Yes, exactly. Matter means SmartThings and the device can understand each other’s messages. So you don’t need custom code to use the device for its basic functionality. (It might have additional advanced features that are not available through the matter integration.)
So do all matter devices work with ST as long as you have a matter controller or thread border router in the ST ecosystem so like no matter what the device is as long as it says matter on it then it will work with any matter controller or thread border router?
In theory, yes. But there are some important details. And you need to have a smartthings/aeotec hub. Read the community FAQ:
FAQ: What do I need to add a Matter device to the SmartThings app? Do I need a bridge router device?
eero
Also I read my eero 6 is also a matter controller/thread border router how does that work? Does it just integrate it into the Alexa app and that’s what you use as a controller?
You’re mixing several different aspects of the matter platform: the commissioner, the controller, and the thread border router are three different things. Again, read the FAQ I just linked to. In the setup you describe, the Alexa app is the matter commissioner. Some eero models can act as matter controllers. And the same ones can be thread border routers. However, note that at the present time, the Alexa app does not yet support matter bridges.
WiFi vs Thread
And back to advice on me buying new devices. So matter over wifi tends to be used for devices that run off main power and thread tends to be used for devices that run on a battery right?
Yes, although it’s not a hard and fast rule, it’s up to each manufacturer to decide for each model which protocol they will use.
Is there any benefit of one over the other besides thread using less power so it’s better suited for batteries or should I stick to matter over wifi for plug in devices and matter over thread for battery devices?
There are pros and cons to both as we have discussed. Wi-Fi is faster and can handle more bandwidth, but uses a lot more power and can support fewer devices per network. For example, Eve makes a very popular smart plug that uses thread. Very well engineered with excellent energy reporting features when used with SmartThings (energy reporting is not yet part of the matter standard, but Eve has a custom integration for smartthings which works out of the box). But it is expensive.
SMART PLUGS AND SWITCHES
I’m looking at now getting matter smart plugs and light switches rather than z wave like I said before so I’m just wondering what I should be looking for or avoiding when making these purchases. And that’s assuming that matter is what you would recommend me to use or would you recommend using just a regular wifi smart plug device since z wave is losing support or whatever and Zigbee could get interference from my wifi
That’s a personal decision, there’s no one best answer. 
A lot depends on how many devices your Wi-Fi setup can support and that varies by both the router you select and the Internet plan you select.
As far as Wi-Fi interfering with Zigbee, it can, but there are also literally millions of people using Zigbee devices successfully. (Zigbee is the smart home hub built into some Amazon Alexa models, for example). There are things that you can do to reduce interference issues, including channel selection and physical location placement.
I certainly wouldn’t be afraid of it, I use a number of Zigbee devices in my own home. It’s just something to be aware of, particularly if you’re having a problem with a particular device. The one device class where I personally stay away from Zigbee is door locks. I have multiple cameras, including a video doorbell, in that area, and I tend to use boosted Wi-Fi there for that reason. But choice is good.
@johnconstantelo has more than 200 devices and mostly uses Zigbee, including for light switches. He may have more to add.
IS THERE ONE PROTOCOL I CAN USE FOR EVERYTHING? AND SHOULD I ONLY GET MATTER DEVICES?
Again, speaking just for myself, I have high hopes for matter, and seeing it is a plus, but I do select each device on a case by case basis, depending on exactly what features I want, and other aspects, again, like the spaces in my home where I use boosted Wi-Fi. I do strongly prefer that my devices work with Apple home, Alexa, and hopefully SmartThings all at the same time. So for me, matter should deliver that, but again it’s been a pretty bumpy rollout. We’re still waiting to see whether matter will really work out.
If you’re only going to use a device with one platform, then I would recommend choosing the best one for that platform with the expectation that it has to justify its cost within three years in case you want to switch to something else later. If you’re going to use a device with multiple platforms, matter should make that easier.
SmartThings is a multi protocol platform, even for the hubs that don’t support zwave. So you’re probably going to have a choice of devices from different protocols for each item you buy. That’s a good thing, but it means there’s no one perfect answer to guide all your purchases. You’ll need to consider the details of each use case. 
A GOOD ENOUGH SOLUTION
If that feels like too much work, and you’re willing to settle for a “good enough“ solution, then my recommendation would be
a) matter over Wi-Fi for light switches If you want switches under $25 each or Luton Caseta if you’re willing to spend more. (Lutron doesn’t yet support matter, but it already works with every major voice assistant, and the engineering is fantastic.)
b) whatever smart plugs work best with your platform of choice. If you want energy reporting, you can’t get that from matter right now anyway.
C) matter lock (and it’s OK if it works via a bridge) unless there are specific features that you want like a fingerprint reader, in which case you may need to choose the model that works best with your platform of choice. Locks are likely one of the most expensive items you’ll get other than appliances and cameras And also the one you’re least likely to use by voice. Matter support will give you a lot of flexibility.
d) matter over bridge for Smart bulbs. I personally prefer the hue bridge, it’s just great engineering.
e) matter over bridge for battery powered sensors of all types. The individual sensors will probably be Zigbee, but they could be Bluetooth or even a proprietary protocol. You’ll have lots of choices. I use aqara, hue, and SwitchBot In my own home, but there are other choices as well. You may eventually be able to use matter over thread in this device class as well, but there just aren’t a lot of choices yet.
f) matter doesn’t offer camera support yet, so for security cameras and video doorbells and baby monitors I’d just get whatever works best for the particular use case and fits your budget. They’ll almost certainly be Wi-Fi, since that can handle the bandwidth that video requires. I personally use a lot of Aqara Because they fit my budget and work well with Apple home, but the cameras don’t work well with SmartThings. (few cameras do, to be honest.)
(BTW, if you want simple, I also like an almost all z wave setup plus a hue bridge for smart bulbs, but I wouldn’t use SmartThings for that. Zooz zbox, Hubitat, Homey, or Homeseer all have better Z wave implementations. I know that’s not what you’re looking for, but I did just want to mention it. )
SO…I DON’T NEED MATTER?
Matter is supposed to make it easier to select and setup new devices. Which is nice, but it’s been a bumpy rollout and there are a lot of glitches yet.
Apple home users will get the benefit of a wider choice of inexpensive devices.
Smartthings users will get the benefits of more local operation And no custom code requirements for basic functionality, Both good things. Note, though, that, even for matter devices, the smartthings app still requires a cloud connection. 
Zwave users don’t get any benefits at all at the present time.
Many people will get a peace of mind benefit and knowing that if they decide to switch to a different platform, or just add an additional platform, their devices that support matter will probably work with the new platform easily, thus making their dollar investment go farther.
For me, because I use multiple platforms and one of those is Apple home, seeing matter support is a real plus right now. But it’s not a minimum requirement for our household. The strength of the individual solution for any one use case is more important to us. Choice is good. 