a-shell uses ios_system https://github.com/holzschu/ios_system/ and can install WASM packages (if they exist?)

iSH emulates 32-bit alpine x86 and can install things from apk

What’s the comparison between a-Shell and iSH? I’ve somehow never heard of the former, but I have repeatedly heard about the latter and used it.

a-Shell doesn't try to run a full linux environment; it just gives you access to a lot of common utilities, which presumably was an easier sell to get on the app store. The fact that iSH Shell allows you to download and run arbitrary code from alpine's repos is what made many think that it'd never leave TestFlight.