what is the signal that interrupts the script

SIGINT (interrupt signal) is a predefined signal-> it's Ctrl + C
The way I use it ensures, that the program finishes thing() before exiting.
What SIGINT stands for was explained in the example I linked a line above my code...
It was confusing me too, when I first saw it: I thought SIGINT was some sort of integer.

do you use or recommend using Merry?

No XD

SQLAlchemy

This is something I have to start studying

It's really worth looking into. My SQL is ass. But I understand (for the most part) what SQL does and that's kinda important.
SQLAlchemy isn't necessary to use SQL, but makes it neater Python code.
I'll write a big long series about how I build things and why and I'll be using it, and once you touch flask and flask tutorials (by the same guy I linked the error video from) you can't avoid it.

I am not using SQL because it is fast (which it is), but mainly because the built in serialization saves me so much work. It's also easy to backup.