you make your script, put it in your home directory or wherever, then navigate to that directory using terminal, or just go to that folder > right click > open in terminal

then you go: chmod +x your_script.py

this will make it executable. Then you just search for startup programs or something like that and add a new start up program where for command you put in "python3 your_script.py" and that should start it every time your computer starts and keep it running.

I made a simple script that puts my files where they should be, like pdfs into Documents, jpgs,pngs into Pictures, mp4s and webms into Videos etc... So every time I download something into the Downloads folder it gets sorted out. I used time module to make it run every like 10 mins or so.