These services are implemented in different parts of Fastly's production stack, but they share the same global infrastructure footprint and a lot of the same people/teams are involved with both.

would that be true for providers that aren't Fastly? why would there be a different implementation for non-fastly? or are they the sole providers for private relay?

There are other providers of the second hop proxy for iCloud Private Relay, Fastly is one of them.