My question now is, can i import the related wallet with that 12-word on for example a hardware wallet?
Maybe but not directly
Perhaps, but not directly the 2014 Blockchain.info wallets did not use the BIP39 standard used by hardware wallets such as Ledger, Trezor, or BitBox, but rather a non standard 12 word format the 12 words cannot be imported straight into a hardware wallet you would have to manually move the money to a hardware wallet after first restoring the wallet with Blockchain.com
And how can the Bitcoin-Network even recognize what adresses etc are behind thos 12 words if its created on a completely different plattform?
Nothing is recognised by the Bitcoin network itself the 12 words are merely a deterministic mathematical method for recreating your private keys regardless of the platform, the 12 words are used by your wallet software (not the Bitcoin network) to recalculate the exact same private keys and addresses the wallet software can replicate the addresses as long as it comprehends the original technique
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, provided it is properly restored In 2014, Blockchain.info primarily used legacy P2PKH addresses, which begin with "1." All addresses and balances including any legacy addresses and the private keys that go with them, should be recovered when you use the right software such as Blockchain.com today