What is the location of local MBOX files

Mail.app stores its data in ~/Library/Mail folder.

I use method a) to copy it to another Mac.

a) zip the ~/Library/Mail folder and copy it to the other account.

b) Or optionally in Apple Mail > Mailbox > Export Mailbox... But for some weird reason Mail.app can then export only up to 2.15 GB mailboxes so old messages are silently ignored! BTW, remember to choose "Export all subfolders" option, if necessary!

Then in another account:

Mail > File > Import Mailboxes... > Apple Mail > choose that ~/Library/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". Right-click and you might delete "Import" folder (it has gmail mailboxes that are online). YMMV.

The easiest way is to export whichever mailboxes you want, and save them in a location of your choice. This will

Control-click and choose Export Mailbox...

I don’t think it has to be done individually. You can do it to a mailbox folder and export the whole thing in one go.