If a plugin has a github repo and 95% of the github repo is open source but the remainder of the library is not, what does that mean if the plugin is on Fab under the Standard license?
Does the Standard license take precedence or?
Not a lawyer, this is just how the two licenses usually split. Neither one automatically wins.
The Fab Standard license covers the package you download from Fab: the plugin as shipped, the closed remainder, and the use terms (seats, redistribution, whether you get source). It does not reach into the GitHub repo and relicense files that were never part of that package.
The GitHub license covers only the files that are actually in the repo. If 95% is MIT or Apache and the rest was never pushed, those missing files are not open source. You cannot treat the Fab build as MIT just because most of the repo is.
Where the same file exists in both places, the copy you use follows the place you got it from. A file you cloned stays under the repo license. The same file inside the Fab plugin is used under Standard, because Standard applies to the package as a whole. If those two conflict on a file you need to ship, the safer reading is the stricter one for that copy.
So Standard does not override the open-source license on the GitHub code, and the open-source license does not unlock the closed part sold on Fab.
Check three things on the listing before you depend on it: the Fab license text itself, the repo LICENSE plus any README note about what is excluded, and whether the closed part is a precompiled binary. For a shipped game, Fab Standard is what Epic and the seller will look at for the plugin you installed from Fab.