OEM configs have never been enough to implement everything they need in Tor Browser. They eventually started their uplift effort [1], to upstream all the patches and features they've added to it, so that they can possibly just use OEM configs.
Looking it up, I think the more accurate term is actually "partner repacks." [1] They're versions of Firefox shipped with different default settings and/or an extension included by default at install time, IIUC. Though reading this rather opaque page [2] it doesn't seem to explicitly preclude patches to the compiled code, though it seems like they would forbid that.
I haven't looked into it in a while, but last I knew you had to have MoFo sign-off on a bunch of stuff, including code changes (i.e., you're only allowed to use the Firefox branding if you get MoFo sign-off on those changes; you can always redistribute it without the trademarks).
Project Fusion is a superset of that effort.
[1] https://wiki.mozilla.org/Security/Tor_Uplift