Where exactly are the mail folders "on my Mac" on my time machine backup?

I occasionally do this from Carbon Copy Cloner backups or by archiving ~/Library/Mail folder. ~/Library folder is hidden by default. When booted to that macOS it can be revealed by holding down Option-key when selecting Finder > Go > Library. Or, when booted to another macOS volume by revealing hidden items via Shift-Command-. (that's a period at the end).

So with Time Machine restore that ~/Library/Mail folder (I don't use TM so I don't know the details for that).

Then import that Mail folder to another Mac or to a new macOS install:

Mail.app > File > Import Mailboxes... > Apple Mail > choose that copied Mail folder > choose all Items.

On the left-hand-side under "On My Mac" > Import, ..., look for and move "Local" and all mailboxes inside it up under "On My Mac". In my setup I right-click and delete "Import" Mailbox (it has gmail mailboxes that are online). YMMV.

Follow @Matti Haveri's advice by using the Finder to navigate to the folder. Then click on the Time Machine icon on the menubar to open the TM interface where you should be able to select the folder to restore.