The public key is used within a function by the person sending the message, and even someone that knew the function and the public key wouldn’t be able to decrypt the message, because doing so would require knowledge of the original prime numbers which they couldn’t work out unless a computer spend years factoring the public key.
My only other bit of confusion:
If someone used a public key to encrypt the message “Hello”, maybe it would spit out something like Gh5bsKjbi4
If someone else sent the exact same message I assume the outcome would also be identical, and therefore it would be possible by using common phrases to work out what was sent? I could type messages like Hi, Goodbye, Hola until I got to ‘Hello’ and realised it was the same output.
However I assume that a message like ‘Hello, how are you?’ would result in a completely different output (despite Hello appearing in both) and thus trying to work out any messages in a brute force way like this would be pointless.