Every launcher exports RETROBIOS_PAD: gptokeyb, muxterm, evmapy or miyoo, so the interface can name the buttons that reach it. The wiki lists the values, and the tests check each launcher's.
RetroBIOS for muOS
Copy RetroBIOS.muxapp to the card, open it from muOS's archive manager to
install it, then start RetroBIOS from Applications.
It checks the BIOS folder (/run/muos/storage/bios) against the files its
emulators read and installs the missing ones: downloaded with a network
connection, or from a pack copied to the root of the card without one.
It runs in muxterm, muOS's terminal. With PortMaster installed, gptokeyb2 turns every button into a key and muxterm is told to leave the pad alone, so each button is sent once: D-pad to move, A or Start to choose, B or Select to go back, X to tick a system, Y for the options, L1 and R1 to page. Hold Select, then press Start to quit. Without PortMaster, muxterm reads the pad itself (Y then ticks a system) and Menu closes it.
Not tried on a device yet: the icon, and whether muxterm honours SDL's controller allow-list as SDL 2.30 does.