codex

I use a GoPro 9, and a Maverick 3 classic for my videos on YouTube. I edit in Final Cut X. What codex should I use. I guess I have to use MP4 because of GoPro.

Native GoPro footage can be computationally expensive during edit. I'll typically transcode to ProRes before ingest on that stuff.

A fairly large consideration when making this decision would be the hardware you're using - your Mac's specs, whether you're using internal or external storage, any external storage specs, etc.

16 GB ram.
I guess I wasn't clear on my question. What codex should I film in? 264or 265?
GoPro 9 Hero has to be in MP4, (There is no .MOV). I have a choice in codex 264 or 265 . Should I record Raw footage on GoPro and DJI Maverick 3 classic in 264 or 265 ? Thank you. I am not worried about sharing 4K to QuickTime for Youtube.

Ah, I see.

In that case I'd do some testing, try both and see which format your hardware is happiest with. Quality-wise, the H256 files will look better at a given bitrate but also carry a stiffer decode penalty.