Time Machine to do a local back up of my MacBook Air, onto an external SSD that's kept off site (at my mum's house). Backups every week when I go to visit.
BackBlaze for a regular off-site backup that's completely up-to-date. I have this on both my Mac mini and MacBook Air.
As for local backups on the Mac mini, it's a bit more complicated.
I have all my important stuff (photo libraries, music libraries etc.) on an external SSD, due to only having base storage on the internal drive. That is then backed up to a 2nd external SSD via. Carbon Copy Cloner rather than Time Machine.
I experienced a few glitches when trying to backup external drives with Time Machine in the Big Sur days and never went back. If I recall correctly it kept backing up the whole drive every time rather than just the files that changed.
I do still use Time Machine to backup what little is kept on the internal drive though, and that backup data goes to a 2nd partition on the same external SSD I use with Carbon Copy Cloner.
A bit complex really, hopefully if I get a new Mac mini with more internal storage I'll be able to steamline things a bit more.