ftiraid

Prefix /64, PC under USG: PC under another router can't receive RA packets, but if the router connects to another IPv6 environment (not USG), the router can send the RA packets without making any changes. I don't know if there is any configuration I need to change on USG for prefix /48?

Yes, I did that and received IPv6 address. And another router's WAN has IPv6 address. But router's LAN and device under the router didn't have the IPv6 address

Yes, with prefix 64, I can receive the IPv6 address, but I have some devices connected to a router that is under USG50, I want those devices to have the IPv6 address also, but I am not sure what the steps are.