Key aggregation combines the participants' public keys with per-key coefficients to stop rogue-key attacks. Each signer first sends two public nonces (this round can be done in advance), then partial signatures, which anyone can add up. Nonces must never be reused or derived deterministically from the message alone; BIP-327 specifies nonce generation, and reuse leaks a signer's key.
MuSig2 is n-of-n. For t-of-n use FROST or Taproot script paths with several keys. Coordination (nonce exchange, PSBT fields from BIP-373) is the hard part in practice, and hardware-wallet support is still limited.