pooya87 basically answered your questions. But to add a small information regarding the second point:
So, if that file is gone forever does it mean my satoshis are gone forever too, or is there a way to get my satoshis back in case that file went missing?
The word seed you are prompted to backup can regenerate all of your private keys.
You can either reimport it into electrum or use any other tool to derive the private keys (e.g.
https://iancoleman.io/bip39/).
You might download the site and store it offline. Once electrum should not be available somehow, you still can regenerate all of your private keys and import them into any other wallet.
This way you are completely independent from any software-/server- provider.
A better option would be to use a hardware wallet (nano s) with electrum.
This way you will be using a BIP39 compatible seed (which does work in any other BIP39 compatible wallet (almost everyone)).
And your private keys will stay in a secured environment, which does negate most attack vectors.