are they forks? That's what I don't get, who controls the merge controls into Chromium's main branch? It's open source, but who actually says yay or nay on PRs getting in? I assume it's Google, and the others are all forks off, but if it makes it into the main branch or not will really decide if it gets adopted

They can, but their very existence increases the Chromium engine's market share and therefore Google's control of the web, allowing them to do stuff like this. Once this is implemented in Chrome then these browsers will just become "Chrome but it can't play netflix/access bank websites/etc" or whatever.

Brave calls itself a fork, which I suppose if its truly a fork, they are cherrypicking patches they can use from the chromium base, rather than recompiling with their own patch set on top

Goggle standard approach to it, is to integrate it so much with other components that it will be a lot of work to disable it, eventually making it impractical.

The right way would be for those clients to switch to gecko engine.