Ethernal will pull available accounts on the workspace server and display them there along with their balance.
If you click on the address, you'll be able to see the transaction history.
You'll be able to use any of the accounts listed here when interacting with contracts.
To add an unlocked account and start using it, you can use the "Add Account" button. No verification is made to make sure the account is available, so you'll need to check that before.
If you set up a private key for an account, it will be used for every read & write request from it.
You can also impersonate accounts from this page ("Add Account" button).
This tries to call
hardhat_impersonateAccount, if one of them succeed the account is added to the list, its token balance will be displayed for detected tokens, and you'll be able to use it to call contracts methods.