Game Pass Ultimate grants access on Xbox and/or PC to a giant library of games. You can stream via xCloud including console-only games, so you don’t have to download everything.
You can convert Xbox Live to Game Pass Ultimate and it ends up saving you hundreds. For example, if you buy 1 year of Xbox Live, you can convert that to 15 months total of Game Pass Ultimate (extra month if you agree to recurring billing).
The limit is 3 years.