pi 3b+ – Bluetooth serial communication server application

I found this app (Serial Bluetooth Terminal: play.google.com/store/apps/details?id=de.kai_morich.serial_bluetooth_terminal) on Play Store and I’d like to use it to communicate with my Raspberry Pi 3B+. I understand that I need some kind of “bluetooth server” running on my Pi and being able to communicate with the mentioned app. However, I’m a complete newbie to bluetooth stack software. I have read these articles and sources:

But I don’t know where to start because I don’t comprehend the basics on bluetooth communications. What I’d like to have is the simplest bluetooth server application (written in whichever programming language) to act as a server for “Serial Bluetooth Terminal”.

The flow I would like to implement is the following:

Client (android app) sends a line of text.
Server reads it and processes it.
Server sends back a response to client, accordingly.
Client starts over.
Eventually, client or server has an option to disconnect from each other.

Can anybody help me to configure my Raspberry Pi OS and/or write that server application?

Any starting point, example code, step by step instructions… would be valid for me. Thanks in advance!