Wallet App and My Credentials

Hi. I was talking with my father about how I was going to start getting some satoshis to hold for the long-term, and he said something I'm not sure about: everyone that's the owner of some of those hot-wallets apps have access to all of the wallets; and that's one of the things that doesn't make him so confident about creating a wallet in a user friendly interface.

(Blue Wallet example)

When we are importing a wallet, we need to write all of the 12 or 24 words in the app and have a internet connection. What guarantees me that the company or the people behind the app don't have any information about my 12 words? Is there somewhere in the code (in case of Blue Wallet, since it's open-source) that shows that this doesn't happen? If someone could explain how this process works in more details, I'd be grateful.

Ty.