The SmartThings /Alexa integration is at present one way. Smartthings will expose its devices to Alexa so that you can use voice control with them or, if they are a contact sensor, motion sensor or a lock, you can use them as the trigger for an Alexa routine (not a SmartThings routine).
Alexa devices do not show up in the smartthings app and you cannot issue commands to them from smartthings.
However, the good news is that an Alexa routine CAN have an echo device as the target, so you can create custom voice notifications there.
The question then is how do you put these two things together?. That is, have an event on the smartthings side trigger an announcement on the Alexa side.
This is a very popular use case, and it’s pretty easy, although you do have to do some setup work.
What you will do is use the Alexa routine to make the announcements. Then you will use a smartthings sensor (either real or virtual) to trigger that routine.
And specifically, if you want to use a virtual device, it’s usually best to use a virtual device which is BOTH a switch and a sensor, so that when you turn the switch on, the sensor looks like it opened. And when you turn the switch off, the sensor looks like it closed. That way you can turn the virtual device on and off by voice or with a smart things automation and the sensor piece will trigger an Alexa routine.
I’m surprised you haven’t found this in your research, as there is a community FAQ on it and it is a very popular use case.
Here’s the direct link to that FAQ. It also provides the information you need to create that special kind of virtual device.
FAQ: Can I trigger an Echo Action without Speaking to It?
In the future, you will probably save yourself a lot of research time if you start with the quick browse lists in the community-created wiki and look in the project reports section. For example, there are lists there for alerts and for voice projects: either would take you to this information. 
You might also want to take a quick look at the FAQ section of this forum, as many common use cases are discussed there. But the quick browse lists are probably quicker because they are more specific.