Welcome! 
Basically, there are three different approaches. None of these is the “best approach,“ instead, different things will work for different people depending on how much time you want to spend on research, on the actual set up, what your budget is, etc.
FIRST APPROACH: WAIT UNTIL SMARTTHINGS SUPPORTS “MATTER BRIDGES” (MOST LIKELY A YEAR FROM NOW)
If everything goes as planned, hands down the easiest way will be to wait until the new industry standard, Matter, is fully deployed and smartthings Itself supports “matter bridges.“ (smartthings currently supports bringing in individual matter devices, but not a matter bridge.)
Why is that the easiest way? Because the whole point of matter as a new industry standard is to make it easy for customers to select devices that will work with what they already have.
You’ll look for devices that have the matter logo. In some cases, this will be an end device, like a door sensor. in other cases, the sensor itself won’t have the logo, but it will work with a “bridge,” which is typically a hub or mini hub of that brand, which does. You’ll add the sensors to their own hub, then you add that hub to smartthings, and it should bring in most of its connected devices with it. (This is pretty much how Apple’s HomeKit works today if you’re familiar with that.)
Why is this the simplest? Because the matter bridge will let you use the full functionality of the end device in its own app, and use some of the functionality in the smartthings app, with a pretty easy “onboarding“ process, and almost no research required. And no special code needed. 
This should open up a quite large set of inexpensive candidate devices
All of which sounds great, but we aren’t there yet. The entire industry is in the middle of this huge transition to matter and it’s taking longer than anyone thought it would. You should definitely not buy most devices ahead of time because we don’t really know what functionality is going to be delivered in the future. As I mentioned, smartthings itself still doesn’t support a “matter bridge.“
So if everything works as planned (and I know that’s a big if) you should be able to save a lot of time, money, and hassle by waiting. But it does require waiting.
Here’s the community thread that discusses the new matter standard from a smartthings perspective if you’re interested in that:
Matter - smart home connectivity standard (formerly Project CHIP)
SECOND APPROACH: PRETEND THIS FORUM DOESN’T EXIST, AND ONLY BUY DEVICES WHOSE —EXACT— MODEL NUMBER IS LISTED IN THE SMARTTHINGS APP FOR YOUR LOCATION.
SmartThings (or certified manufacturers) provide “stock” integrations for dozens of models, but not all models for all brands. Devices with stock integrations will be listed in the smartthings app, but you have to check the screen which shows the exact model numbers.
For example, as of the date of this posting, here is the list for Aqara for the United States.
Note that it says there are stock integrations for the motion sensor T1 and the door and window sensor T1. And that’s it. Not for the motion sensor E1. Not for the motion sensor P1. Not for the presence sensor FP1. Just that specific T1 or the specific model of high precision motion sensor. Even if the sensors look identical on the outside, and seem to have the same list of features. 
First rule of home automation: “ the model number matters.”
On top of that, it’s important to understand that lots of devices that have a stock integration may not “expose” all of their features through that integration. A door sensor may only have basic open/close when viewed through the smartthings app, even if the device itself has other features like a tamper alarm or adjustable sensitivity.
(This is true even of the Aeotec devices.)
If the model number is listed in the smartthings app, it will be easy to add, you won’t need custom code, but it may have limited functionality that doesn’t match the product description or what’s on the box.
Some of the brands that have official integrations, including aqara, ikea, Meross, and Sonoff, are quite inexpensive, so there are a range of choices.
Research is a little more difficult than it will be with matter-compatible devices, but not too bad. And setup should be easy.
THIRD APPROACH: USE CUSTOM CODE TO EXPAND POSSIBLE DEVICE CANDIDATES AND GET MORE FEATURES
The third approach, and the one that most people in this particular forum take, is to take advantage of custom code which has been created by various community members to allow integrations with devices which either are not on smartthings’ list at all, or are, but don’t expose all the features of a particular device.
This then enables you to choose from hundreds of devices, and get a lot more functionality from them. But it does mean you have to research in order to find custom code and install it.
Beginning in 2023, installing custom code actually became a lot easier than it used to be. Now, in most cases, you can just follow a link that the author will give you, subscribe to their “channel“, select the driver you want to download to your own hub, and it will happen soon there after. After that, you can go into the smartthings app and assign a particular device to a particular custom driver, and you should be all set. More devices, more functionality.
So it’s a very popular choice for power users, but there are some extra steps, and it is a little more work and can be a lot more research than the first two options.
People quite commonly start a new topic here and ask for recommendations for devices to meet a particular need, and that’s when other community members will recommend not only a specific device, but often a specific custom driver. But if that process feels like too much work, then one of the first two approaches might be a better fit for you.
I hope that helps, and welcome again.
