New to green wallet

Hello everyone. I just downloaded blockstream green wallet on my phone. Just a few questions.

1. Is hot wallet really safe? Do I need a cold wallet for bitcoin? Also I found that the seed was stored in the application. Is it fine?

2. I could make several accounts in a wallet. Standard singlesig, multisig 2of2, ... What are the differences? I set up 2fa, including google authenticator, my phone call and email.

3. I've heard of passphrase, which is a code after the seed. When and how should I use it? I use pin to login, and use seed to retrieve my wallet. I never use a passphrase before.

Any answers or suggestions are appreciated. Thanks!