Once you have created an account (a public-private-keypair) in Polkadot{js} extension, that account can then derive a bunch of addresses, one for each Substrate-based chain e.g. the same account can have an address for Polkadot, a different one for Kusama, another different one for Acala. But they all share the same private key and public key. As an analogy, they share the same username and password, but the account display name are different for different blockchains you interact with.