Most spam starts with a sign-up you barely remember. If the address you gave was temporary, the spam goes nowhere.
The address is only valid for hours. When marketing starts, there is nobody on the other end. There is no inbox to filter and nothing to unsubscribe from.
An alias forwards to your real inbox, so it still needs filtering and can be switched off. A temporary inbox never forwards anything: mail stays here and then disappears. Choose an alias for accounts you keep and a temporary inbox for one-off sign-ups.
Yes. The confirmation arrives in the inbox, you click it, and that is the last you hear from it.
Anyone who knows the address can open its inbox, so never use it for anything sensitive.