Why bech32 address always starts with bc1q? I understand that it starts bc1.

But 'q' is not what I understand. Can someone explain to me?
q is the Bech32 encoding of the number 0. For a segwit address, as defined in BIP 173, the number immediately following the separator is the witness version. In this case, bc1q, the q means it is a witness v0 address, which is the only type currently used but allows for easy future addition of new versions