1 - you register your keys with your Bank, and it communicates the Central Bank. It can be your "ssn", your phone number, your email or some random id.
2 - to pay somebody, just provide the key. Your institution will look up in the Central Bank database who'll be paid with that key (the destination: Bank, account number)
3 - your Bank withdraw the funds from your account, the destination Bank deposits it (there's a tight time limit), the Central Bank does the compensation among banks.
4 - there's no disput: once you authorized it in your Bank, it's done.
If your Bank provides you with credit, it's a commercial relation between you and your Bank. Pix is just a transfer from your balance, it doesn't involve credit lines and so on.
So, in the end it's simple: money transfer with tight rules made by a Central Bank and obligatory adoption from all Banks.
The keys are the most relevant aspect to the clients: your phone number, your "ssn" ( In Brazil it's called CPF and it's public, it's not a secret number). Just like WhatsApp made it easy by using phone number instead of some login.