As OmegaStarScream says, the new account will be under the same seed phrase.
Your first segwit bitcoin account is derived at the following path: m/84'/0'/0'
If you create a second segwit bitcoin account, it will be derived at: m/84'/0'/1'
If you create a legacy bitcoin account, it will be derived at: m/44'/0'/0'
If you create a second legacy bitcoin account, it will be derived at: m/44'/0'/1'
And so on. Segwit accounts are at 84', legacy accounts are at 44', and the the third number is incremented by 1 for every new account you create.
Your "Add account" button is grayed out because your current account is empty. Trezor will not let you create a second (or third, or so on) account until the previous account has been funded.
Now this is clear to me in the case of bitcoin.
But how does this work in the case of different crypto currencies and completely different blockchains?
When creating an account on any HW wallet (can be Trezor or from any other manufacturer), seed-phrase is generated. One single seed-phrase, but on a device we can create hundreds of wallets. I mean wallets with different crypto currencies standards: BTC, ETH, XMR, etc. How is it technically arranged?
Imagine the situation: need to restore from HW wallet (from device with the same seed-phrase), which stored crypto currencies with different standards (for example, electrum with BTC, myetherwallet with ETH, Monero GUI Wallet with XMR). Is it enough to enter into each of these wallets (electrum, myetherwallet, Monero GUI Wallet) that wash and the only seed-phrase from HW wallet?
Can I get at least a superficial explanation of how it works?