I want to learn more about adding keys (signers / signing devices) to Keeper

The Add Keys section is where you setup signers for a specific vault.

You can use a combination of hardware and software signers to setup your multi-key.

You could also use a previously setup signer being used in a different vault, in a new vault. However this is risky as loss of access to the signer would mean loss of one key for both vaults.