Once you have created an account with Polkadot.js.extension, you basically have a wallet address for every supported network (i.e. a DOT wallet address, a KSM wallet address, a CFG wallet address etc.).
But all of these wallet addresses are just different representations of the same public-private key.
You can visualize it like this:
You can also see the different wallet addresses in your account by going to the Polkadot/Substrate Portal and changing the network for the wallet address you want to see (like explained in my first answer). Just make sure to change the chain to that network (or “Allow use on any chain”) in the dropdown menu in the extension as well.