This is a raw theoretical question.

I shall be grateful if you explain me "what happens inside of hardware wallets" :-)

Say, you are setting up a multicurrency wallet.
Your seed (24 words) is set up and your wallet is ready.
You can see 10 (just for example) different cryptocurrencies in the same wallet. OK!
But all those 10 cryptocurrencies belong to 10 different blockchains.
How come that you have only 1 seed (the same 24 words) for 10 different blockchains?

May be it works like having the same login/password for 10 different email boxes?

But imagine, you set up the same seed on another hardware wallet which suports, say,
20 cryptocurrencies.
You are ready with your set up, and you can see now 20 cryptocurrencies in your wallet.
It means 10 more blockchains.
But how these 10 new blockchains were instantly added to the wallet with the same seed (the same 24 words) when this seed was never known to them before? This seed didn't exist for these blockchains before your set up the new wallet.

If it works like with email services, then
I can chose 1 login/password and use it for 10 or 20 or 100 email services like gmail, outlook etc.
But I need to register every email box with this login/password before using it.

Or is it so that when you set up the new hardware wallet with 20 currencies, which means 10 more blockchains (in this example),  it will "register" the old seed in new blockchains automatically during set up?

Could you please explain how it works.