Frankly, a fixed "Internet" Service Provider that doesn't provide you with a router (and leaves you the possibility to use your own) and a /48 IPv6 prefix should have no legal right to call that service "Internet" (in a similar way that Internet neutrality has to be legally respected) :

https://www.ripe.net/publications/docs/ripe-690

(Mobile cellular Internet seems to be harder, but is there any reason why user's cell-'modem' can't handle the routing of successive /64 connections ?)

If I have a second phone (e.g. Android( running a hotspot, and I enable a VPN on that phone, will the hotspot / shared internet also go through the VPN, or will the hotspot bypass it?

In the future I think the tech savvy will be doing exactly that - pinefone as a wireless hotspot with wireguard to a anonymised proxy.