A note and reminder about Relays in general are that they are more of a convenience in the protocol than a hard requirement. The “firehose” API is the exact same on the PDS and on a Relay. Any service which subscribes to the Relay could instead connect to one or more PDS instances directly.
Isn’t this the source for the relay? https://github.com/bluesky-social/indigo/tree/main/cmd/bigsky
And even then the readme says:
And the PDS source code is here: https://github.com/bluesky-social/pds
EDIT: The PDS source is actually here: https://github.com/bluesky-social/atproto/tree/main/packages/pds the other link is for self hosting.