how to use my express backend for my app?

For school project, we have to make an app (in Kotlin) and a web application (in Angular) using the same database. After lots of research, we decided to use a MySQL database together with a restfull api using express.

We found out how to use angular and express, this was quite easy. But now I've been trying to use Kotlin and express. It is here that i'm stuck.
I have been searching around on the internet, but nowhere did I find a good explanation for my problem.

Can anyone help me on my way to the solution? Or am I doing something completely wrong?