TorTaughtA v3 onion address is 56 base32 characters before the .onion suffix. The string encodes the service's Ed25519 public key plus a checksum and version byte; the public key being baked into the hostname is what makes spoofing computationally infeasible. The TorTaught walkthrough does not require you to understand the cryptography to use the platform safely; it does require that you trust the verification step (the PGP signature) over your own visual inspection of the URL.
The pinned reference at site16.jrussell.org/v3-onion-service-specification has the full protocol-level reference for users who want to dig in. For day-to-day use, the only thing that matters is: a real Nexus mirror is 56 chars, sits on the homepage roster, and serves a verifiable PGP signature.
Verified working Nexus Market mirrors
Three v3 onion addresses currently serving the production market, signed under PGP fingerprint 0x7F2A0A9D. Use the Copy button; never retype an address.