So sorry if not really proper place on asking, but other sites has no the working/correct answer..
How do we copy a Twitter video into PC storage using a chromium-based browser on Linux?
Sometimes I've needed to download videos, there are numerous video downloading desktop apps in the repo and AUR that might or might not help, (eg. clipgrab, youtube-dl) plus browser addons and extensions. Some work on some websites, others work on other websites, most don't seem to work at all. Some work, then when you try to use them again after a few weeks they have stopped working. So basically you just need to try some things and see what is currently working on the site in question, there isn't a definitive answer, expect a lot of failures. Also some options may limit you more than others in terms of download quality, and I think it was clipgrab iirc that sometimes adds padding to the end of the video I needed to edit off using ffmpeg.
If all else fails you can use screen recording software like obs or simplescreenrecorder.
Not ideal but better than nothing.