Bitcoin Core does not support BIP39, or any other seed word standard. You are correct that this implies backing up wallet.dat and keeping it safe directly. This is a good idea in any case, even if there were a seed phrase, as the seed only helps recover the keys, but not information such as labels you may have assigned to transactions, or unconfirmed ...
A memonic seed is used to generate addresses used for Bitcoin, your wallet.dat generated in your config is basically that
For example, a memonic seed phrase would be 24 words that look like:
suffer adapt adult also depart ect. Ect. Ect
You can generate your own and play with it at https://iancoleman.io/bip39/ it uses the bip39 format and explains everything
This can't be prevented or detected.
But if you're considering the possibility that your wallet itself is under attacker control, you have bigger problems. It could just send your coins to the attacker directly, without needing to trick you into creating a backdoored seed.