- if you're talking a private/local prefix, you can use tools like this to generate one: https://unique-local-ipv6.com/. Otherwise DHCPv6 and SLAAC will ensure no collisions for the most part.

- Use global/public addresses on all your devices (using something like prefix delegation) or use NAT.

- Same as IPv4. Prefix delegation will let your ISP assign you multiple networks, and then most routers will break these up into /64 networks for each of your VLANs.