There are two official methods of integration with Alexa, and three more unofficial ones. Each can do different things.
1. Smart Home Integration
The first is the standard one that you would expect. Ask Alexa to have smartthings turn on a light controlled by SmartThings. This works very well.
In addition, there is a nice trick that many people use which is that you can have echo turn on a virtual switch, And then have that virtual switch coming on is a trigger for pretty much anything that smartthings can do with an automation. So you can change the location.mode , unlock a smart lock, etc. If you are using the new V3 app, there are some limitations because some of the security functions are withheld from this feature. But other than that, it’s very useful. 
(If you are using the classic V2 app, you can also access the security features, but since that app will be going away eventually, you shouldn’t count on it.)
This FAQ Is for the classic app, but the same basic principle applies with the new V3 app, it’s just that, as mentioned, some features like setting security.mode or not available with the new app.
2. Integration with Echo Routines
Echo Has its own routines feature, different from smartthings. With this feature you could have echo speak a custom phrase, start playing news, play the weather, lots of different things. At the present time, echo routines can be triggered by a spoken phrase, an echo button, or some contact or motion sensors.
The really cool thing is that on the SmartThings platform you can create a virtual sensor and turn it on and off. So that means you could have any event which smartthings can you recognize turn on the virtual sensor and trigger the echo routine that way. This is a very popular way of getting custom echo announcements for things like a particular door opening or even someone arriving home. See the community FAQ,
FAQ: Can I trigger an Echo Action without Speaking to It?
For example, if you were using JUST The official echo feature, you could have it say “the front door is open” anytime a sensor on that door opened. But what if you didn’t want that announcement between 11 PM and 7 AM, because you would have your security system turned on then anyway, And you don’t want to wake the baby if one of the parents is coming in late. In that case, you could put logic on the SmartThings side to not open the virtual sensor during those hours.
OK, those are the two official methods. The first one lets you have echo turn on a virtual device and have that device coming on trigger any smartthings activity that you can automate.
The second one lets you use whatever logic you want on the smart things side to turn on a virtual sensor and then have that trigger an echo routine. Particularly useful for custom voice announcements and for partial integration with devices that do work with echo but don’t work with smartthings.
That brings us to the custom methods. These are all very popular with power users, but requires significant technical skill to set up. For example, you will have to sign up for an Amazon developer account. One is called EchoSistant, another is called “Ask Alexa,” and a third is “Echo Speaks.” All 3 will let you do all kinds of things that the official integrations don’t let you do. It’s best to just look at some of the examples to get the idea. They have slightly different features, so you need to pick the one that meet your own needs best. These are free code developed by community members, although they may suggest you make a donation if you like the work.
But again, set up for any of these is going to be pretty complicated if you don’t have strong technical skills. There will be lots of people who will be glad to help you, but it’s definitely not the same as using official integrations.
See the posts below for links to topics about “echo speaks.”