Best practices multiple Alexa/Assistant/ accounts +hubs

Trying to make sure I'm still setting things up in a sensible way. I live in a house where one apartment is segregated from the rest of the house. That apartment has its own SSID, it's own C5, Lutron, SmartThings, Hue, and accounts for the above. The rest of the house duplicates that, except no Hubitat. Technically it's the same internet connection, but each network is on its own vlan.

My big question is what is the best way to handle this setup where things like calls between Alexas and Google Assistants would work, but retaining a degree of separation where everybody can't just control every light? Particularly the concern is so that the rest of the house can't control things in the one apartment.

Currently it's set up where my Google Assistant and Alexa is hooked to Hubitat, whereas on the other network it's their Alexa hooked to SmartThings. In Google Home the house vs apartment are listed as different homes, though I am a member of both. It seems that I therefore am able to control whatever device from whatever home speaker, but that they may not be able to control mine (not fully tested, but seems to be the case and I'd like to keep it that way broadly).

More generally, is there a common solution to making devices only controllable by specific people? IE Jack's bedroom lights only Jack can mess with?

Always open to learning! Thanks.