My question now is, can i import the related wallet with that 12-word on for example a hardware wallet?
Yes but why will you do that? No need of buying hardware wallet if you want to import online seed phrase on it. Best is to create another seed phrase on the hardware wallet and send the coins there.
And how can the Bitcoin-Network even recognize what adresses etc are behind thos 12 words if its created on a completely different plattform.
Every transactions is existing on bitcoin blockchain and wallets can easily recognize that.
Also last but not least, as far as i can remember blockchain had wallets with privat keys back then, meaning in case if i restore the wallet which contained a legacy adress it will be also restored?
Yes.
But why not crate a new seed phrase on the hardware wallet and use Electrum to send it there.