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?
Try to cycle it once every two weeks or something. Unfortunately there isn’t much of a right answer to your question, there are advantages and disadvantages of both approaches. This is a good read on the subject: https://batterycare.net/en/guide.html