21" came in 2 variations
9400m GPU which is soldered onto the logic board
ATI HD 4xxx GPU cards
Only ATI HD 4xxx variant on GPU Card can be upgraded to metal GPU card. All 27" 10,1.
These do run pretty well on OCLP Big Sur with a couple of customizations ( link )
GPU Selection, Installation are all basically identical to 11,1
21.5" of 10,1 is LVDS ( LVDS eDP Explanation ) so must use LVDS vBIOS
27" 10,1 is eDP so use eDP vBIOS.
Note this version is EG1.2. If using non EG version of this file ( link ), then will encounter milky screen error mentioned here under Spoiler : Known Issues ( link )
Note the 3 VRAM variations of M5100 on 11,x. But the 3 variation of vBIOS doesn't exist here for 10,1. TODO : Need to confirm not necessary to have VRAM specific vBIOS.