how does this work in practice ?

is there some kind of signature you can use to ascertain that the code you have is what's actually on the device ?

Is it possible to do the same for the SIM (as I understand it, it is its own microsystem and intelligence companies have already _at least_ attempted to insert their code in there) ?

what about play services or its equivalent ? These are often closed source and have lots of system permissions (since they handle lots of capabilities, they need a broad access).

edit : for clarity, I don't think it is a bad idea, I just wonder how doable it is.