notebook is a 10 year old macbook pro without macos I installed xubuntu 24.04 in. It comes with an embedded battery.
First notebook I bought, not from apple, had a removable battery. The vendor told me to maximize its life I shouldn't plug the battery in, unless I need it (like for traveling). This way, I've managed to keep the original battery in good working condition for 8 years so far.
Back to the macbook: I cannot remove the battery and constantly loading it to up to 80% and discharge it up to 20% seems ridiculous. Furthermore, this would deplete the battery even faster, I believe.
What can I do to spare the battery as much as possible?
When MacBooks are plugged in, they get their power from the charger. They are not simultaneously draining and charging the battery in general, unless they need more power than the charger can provide (unlikely unless you are using a charger with lower wattage than the official charger that came with your laptop).
I was not able to find an official source on this from a quick search, but if I remember correctly, this should be true for any moderately recent MacBook. Maybe any MacBook at all, since they only started making "MacBooks" in 2006 and then tech hasn't changed much since then.
Personally, I leave my MBP plugged in during use whenever possible, and I typically unplug it at the end of the day. You don't need to unplug it, but hey, it's a good idea to unplug anything that doesn't need to be plugged in, just to save power.
so when the battery is fully loaded a controller tells the mobo to stop loading the battery and to use power from the charger to simply power the notebook, is this so?
Correct.
Batteries will still lose charge very slowly, so at some point the battery controller will top itself back up. This is nothing to worry about, and I'm not sure macOS (or Linux) will every display the true charge level of a battery. I believe there is some wiggle room built in at the firmware level.