The title should really say it all: If I backup my 24(+1) Seedphrase from my Ledger,
Bitbox, Trezor or whatever => Am I also backing up my Altcoins?
Trezor, Ledger Nano are HD wallets, the seed phrase generate a master private key, also a derivational part to Bitcoin and altcoins they support. You back up all the funds (Bitcoin +Altcoins supported) with just the seed phrase.