b) when using hidden wallets, how is the passphrase generated within the device when multiple wallets are used? Is the seed related in this process? Because if I use the seed to recover my wallets, the passphrase couldn't be generated unless you put it explicitly, so it'll be impossible to access my wallets If I only know the seed, therefore all my BTC will be lost, aren't they?
The passphrase is like a 25th word of the seed. Every passphrase gives you a different wallet, and if you forget the passphrase (or lose the seed) the BTC stored in the wallet are lost.