First of all, there are a set of requirements in the base hardware and the Zoom version for backgrounds. The biggest issue is that you need the latest Zoom to work without a green screen (as that is harder to mask out your face from the background). And that you need a fast machine for 1080p backgrounds (no MacBook Air M1's allowed).
If you just try to export your video with Mac Photos with Export as 1080p video (Zoom handles only 1080p and below no 4K), you will get an "unsupported video format" message which seems like some not well-understood bug since August 2021. And there are some magic length requirements that are not documented, but in looking this, the main issue seems to be pretty simple. Zoom does not support X.265 also known as HEVC encoding so unfortunately you need two steps:
- Take your video and then to to Mac Photos and click on it
- Drag the video from Mac Photos to the main window of Handbrake and this will open it
- Click on Start to start the transcoding using the Preset Fast 1080p30
- The resulting MP4 file should be importable in Zoom/Preferences/Background & Filters
- Click on the plus icon in the right center and then Add Video from ~/Movies