A matter of trust
Both VPNs and Private Relay require you to transfer trust from your ISP to the service in question. When you log into your VPN account, the provider gets your details and can see everything you’re doing online, making it theoretically easy to log all your activities. Mindful of this, VPNs have always played up their privacy, with no-logging policies. Major providers have signed up to the VPN Trust Initiative (vpntrust.net) to establish baseline practices for security, so make sure your provider is one of them.
Private Relay attempts to get around this major issue of trust by forwarding your encrypted data through two servers from different providers. This separation of knowledge is designed specifically to protect you: the Ingress Proxy knows who you are, but not what content you’re forwarding, while the Egress Proxy knows what you’re doing, but not who (or where) you are.