Why do Bitcoin addresses start with 1, 3, bc1q or bc1p?

Addresses beginning 1 are original pay-to-public-key-hash; 3 wraps a script, often an older multisigMultisig (multi-signature)A wallet whose coins need signatures from several keys, such as 2 of 3, so no single key can spend alone. or a compatibility format. Both predate SegWitSegWitSegregated Witness: a 2017 upgrade that moved signatures into a separate part of each payment, making room for more payments..

bc1q is SegWit (bech32), cheaper to spend from and better at catching typos; bc1p is TaprootTaprootA 2021 upgrade that added Schnorr signatures and addresses starting bc1p, making complex payments cheaper and more private. (bech32m). All four are Bitcoin mainnet — the sending wallet just needs to support the type.