What is the difference between seed phrase and private key?
The seed phrase is a 12 word that can be used to get your private key while the private key is derived from your seed phrase. If your seed phrase is multiwallet like trust wallet then you can used it to get your private key from each network like eth, btc. Basically, your seed phrase is like original and the private key is the clone if you put it that way and both can access the wallet.
What if you only have the private key but not the seed phrase?
You can also import your wallet to a wallet provider that also support private key. Some platform only use seed phrase and some used both.