

the way it works is that the veracrypt container basically contains 2 encrypted partitions. if it can’t decrypt the first one with the password, it will try the second one, but always pretend to try both so that the time it takes to unlock it does not give it away. by writing to either, you risk overwriting data in the other one (except that you can input both the hidden and main partition passwords and it will make sure to keep the hidden partition unaffected), but otherwise both partitions are fully functional
plaintext is the unencrypted form of data. encryption produces ciphertext. encrypting the same data with the same key twice results in the same ciphertext, unless additional steps were taken to insert additional data that does not match (like a nonce) to the plaintext