This tutorial is for a2dp only. I've moved to pulseaudio bluetooth meanwhile to use other features automatically.
Compilation instructions for bluealsa a2dp are on the repo page.
Make sure user PI is in the right groups for udev, pulse access, etc (audio, bluetooth, pulse-access, plugdev)
Disable onboard sound, it sucks (comment out/remove the line dtparam=audio=on in /boot/config.txt) and use an external USB soundcard (I just got this one https://www.aliexpress.com/item/Ugreen-USB-2-0-to-3-5mm-Audio-External-Sound-Card-Microphone-Earphone-Speaker-Adapter-for/32507625943.html and it works fine. Previously I used LX-3000's soundcard I had around).
autobt.py goes in /home/pi/autobt.. Technically, this is the place you can integrate stuff like on line #4 where you can send commands, but you need threads or a way to interrupt and listen to commands (didn't research, don't know python).
You also need to download https://raw.githubusercontent.com/pauloborges/bluez/master/test/bluezutils.py in the same d