I did not find much information about the regtest option for
monerod. What does it actually do? What are the differences between
What does it actually do?
It runs in a "regression testing mode", which allows you to create a fake blockchain (i.e. it is not validated by other nodes), useful only for development and testing.
What are the differences between testnet and regtest?
Testnet is a public p2p blockchain, other nodes on the testnet network validate transactions and blocks.