From my understanding, BIP39/32/44 all use the secp256k1 elliptic curve standard from Bitcoin. But I noticed that cryptos such as Polkadot and Cardano have their own registered coin type path for BIP44, but those two cryptos use Ed25519 as their public key elliptic curve standard, so how is this possible?

Isn't those BIP39 mnemonic words all stemming from 256 bits originally? And Ed25519 uses a different private key size than 256 bits?

I am actually unfamiliar with how Ed25519 works exactly so much appreciated if someone can explain it in a high level overview, and then explain how it's compatible with BIP32/44/39. Thank you.