Enter the email address you bought with. Every key for that address is sent to it again, one message per app. They are the same keys as before, not new ones.
Check spam first, then confirm you're using the address on your receipt. If you paid with a different address than you expected, or it's still not there,a@zigao.wang reaches a person who can look it up.
All of them. It looks your address up against every app sold here and sends a key for each one you bought, so there is nothing to choose and nothing to get wrong.
Two reasons. The ordinary one is a lost email. The rarer one is that an app stopped honoring the key you have: license keys are signed, and if a signing key ever has to be replaced, the app says so and sends you here. A replacement is free and your license is unaffected.