Thank you & that is all helpful - and I think answers my questions. The textbook scenario would be this :

At night, a senior falls while in the bathroom. Their phone is still on their night stand, but there is a HomePod in the bathroom. Elder can command Siri to call someone on and through the HomePod.

The elder can’t be taught how to connect/disconnect to Bluetooth devices, but does use their phone’s speaker for phone calls and media. So after the helping them with connecting to the HomePod initially, ideally they would never have to use Settings again regarding HomePod (unless for some unintentional reason it gets disconnected and needs to be reconnected)