Where are emails stored when I put them in files "ON MY MAC"?

They are stored on your Mac in a folder ~/Library/Mail. If you are missing these emails that is where they would be. However, they are not easy to find. You need to work your way down through all the folders to find the exact email you want.

To restore mailbox:

Open Finder

Click “GO” in top menu bar then click option key

Open mail folder

Open V9 folder

Then find the folders labeled with cryptic titles (at this point you could open TM if you want to go back in time)

Look down through those folders to find a mailbox (note: there may be more than one with same label).

If it contains email you want you can copy to desktop

Go back into mail and import that folder from your desktop

How do I find my emails stored "On My Mac" in the various directories?