Where does Mail store folders labelled "on my Mac"?

Shapley wrote:

Thanks. I hadn't realised the tilde was essential.

Good computing Shapley !

The ~ is the computer short hand for your user—

the long hand version would be for example: /Users/YourUserName/Library/Mail/V7

There are 3 Libraries on the Mac, Files and folder that you have r/w privileges reside in your home folder.

Shapley wrote:

There isn't a Mail folder in \Library (OS Catalina)

From Finder>Go >Go To Folder> copy and paste:

~/Library/Mail/V7