Call unity app from external service

I have an android app, what i want to do, is send messages from the android app to an external unity app and have the unity app do some nice things with it.

As a webdeveloper, i was thinking i could make the unity app run on a webserver, making an api within the unity app, then just send a call to the api and let the api process the data to the unity app.

The problem is, i can not find anything online on how that would be possible.

Any suggestions/tips and help are very welcome!

Also, i should note that my experience with unity and c# is minimum.