You don’t have to sign in to use an iPhone or Android phone, though you to have to sign in to use their app stores. Presumably with the advent of DMA though you can avoid creating an Apple ID or Google account if using a 3rd party store (though probably you’ll need some other account for that store, that’s how it always goes…).

Fe: gaming - yes, this is why Apple had separate IDs for gaming center.. and I think Microsoft does this too for Xbox vs Microsoft account?