Simple answer is because its Base58Check so it starts with "S" as for the public keys there some standards.
- Taproot address - P2TR : bc1p
- Bech32 / Segwit: bc1q
- Script address: 3
- Legacy address: 1
They are public addresses, not public key, not private key.
Private key has some representations: Raw, Hex, WIF, WIF-compressed.
Private key formats (Mastering Bitcoin