The selecting of exec can be part of UE loader and hw user-profile
or this can be also the marketplace platform part, so devs will need to distribute the major-public exec first (basic) to fit most of platform users and additional execs will be downloaded depending on platform profile, like they do for google.play - it is known that it is supported or not for the exact device.
If having a UE loader - also can change the user-profile and ask devs for exact exec, in the manner of support case and vote for hardware, for they was able to know - all the needs, this can be the devs place @ unrealengine , so the engine team will know those profiles too or not if user is not sharing his info. What you need to know - is just the instruction sets and some system id for it, so it wasn’t same user many times from single CPU. And it will appear in case - it is not supported yet and the set differs really hard +5-15 new overall sets.
My thought again is - in some cases they got similar instructions sets, but those similar instructions needs to have some interoperability use-case, for it was like symlinked between to exchange those sets inside of engine or may have some additional optimized compiler and few execs. There is also an option to disable/enable some instructions in-game, automatically depending on current instruction-set, so some functions will be disabled, but the game will work anyway - this needs more engine setup and structured options - and this can screw up many things, starting from lags to hardcoded errors, but in terms of engine structure - this can be done.