On Bitcoin, users interact with Multisigs (operated by ALEX LAB Foundation, with a plan to decentralisation) to lock assets to be bridged ("source asset"), and on L2s to receive the L2 asset ("destination asset").
Additionally, users on Bitcoin may provide additional data (OP_RETURN) to trigger certain smart contract interaction on their behalf automatically by Bitcoin Bridge.
Multisigs are Bitcoin wallets that are operated by multiple signers. In contrast to a typicall wallet requiring just one party to sign a transaction, a multisig requires multiple parties or signers to sign a transaction.
ALEX partnered with Asigna who is "a native multisig solution tailored for Bitcoin and Stacks made specifically to offer an extra layer of security to trusted wallets on both ecosystems."