> My biggest problem with “OK Google” is that I don’t know by heart what it can do interactively

Maybe it’s just me but this feels unaddressed and that seems ridiculous.

Why is it so hard for me to find a single, precise location on my phone with an enumerated list of every command Siri or Google can work with?

You can use open source assistant instead like Dicio https://github.com/Stypox/dicio-android and configure it the way you like.