In Firefox for example the shortcut is ctrl+shift+i
So what you can do is to go on her page, use the shortcut above (or a different one if you're using a different browser) to open the devtools, then you try to start playing the video.
You should be able to see what files are being downloaded in the "network" section.
If the video doesn't have a weird encryption, I think you should see a mkv or mp4 file being loaded. If that's the case, you'd just have to use the link to save it directly.
I suppose, but I'm not sure, that Onlyfans use some form of encryption in their videos though. In this case, I think a browser extension specifically made to rip this type of file might do the job.
Anyway, if you want you can send me a DM, I don't want to flood this thread with unnecessary details and since I've never used Onlyfans myself I don't know how the site works.