is there a Tutorial for Apple Mail program?

The Mail User Guide can be found here: Mail User Guide for Mac - Apple Support

And to view the "All Mailboxes" specifically, hover your mouse over Favorites in the left panel, click the + sign and select "All Mailboxes" from the popup.

If "All Mailboxes" is grayed out in the popup, it means it's already added. In which case, hover your mouse over Favorites in the left panel and click the ">" icon, which will expand your list of Favorites.

There may be an Apple books version at app store.

While better 'expanded' online User Guide is here:

• Mail User Guide [ Table of Contents ] - Apple Support

https://support.apple.com/guide/mail/toc