I assume you're referring to aliases (https://support.google.com/a/answer/33327?hl=en) vs. delegation (https://support.google.com/a/answer/7223765?hl=en).
Aliases are alternate email addresses where the email gets delivered to the same email inbox. So if you had a user joesmith@example.com, you could set up aliases jsmith@example.com and joes@example.com, and it wouldn't matter which of those three addresses people sent email to, they'd all end up in the inbox of the joesmith@example.com Google Workspace account.
Delegation allows one user to access the inbox of a different user. So if Joe in our example above was Mary Jones' assistant, Mary could set things up so that Joe could see her maryjones@example.com inbox, and send mail as her, without needing to know her password or log in as her.
Hope that helps,
Ian
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