how do you show email folders that are hidden?

I left the cursor there for a minute or so, but nothing happened. I have my personal account and an iCloud account. One has a number to the right – should I hover to the left or the right of the number? This is in Mail on a Mac, not a phone or a Tablet, yes?

To the very right? To the right of my principal account name, in the column headed 'Mailboxes'? To the right of the (incompletely written in) account name there is a lozenge, with a number in it. The account name would end within the lozenge if the sausage weren'there – but it is. To the right of the lozenge there is a small space. Placing the cursor in that space – to the left, centre or middle of it – produces no response. Beyond the lozenge there is the scroll-sausage. This responds if the cursor moves over it, by changing from paleish grey to almost black.

I've another account, which appears below (after a line devoted to 'Notes' which Catalina appears to have put there). Nowhere to the right of this alternative account elicits any response when hovered over by the cursor.

A little more detail since I first wrote in. All my sub-folders are there (lower down, under 'In My Mac'. They are all empty, apart from a small number of mails I put in them since the disaster.

I found what I think are covert copies of the missing data, in ... Library–>Mail–>V4–>3B8F1D53-5483-41A1-BA81-54C6305CCF6B. The main likely file in this folder is called INBOX.mbox, but I can find no way to open it. Its size, 164 MB, suggests that it contains all the vanished files. If only ...

Thanks for your patience, Tony.

at the side bar, move your mousepointer next to the right side of your accounts name (e.g. iCloud) until you see a little arrow and click on it

To the very right

I wrote : ... lozenge if the sausage weren'there – but it is

I meant : ... lozenge if the lozenge weren'there – but it is

Sorry!