how empty mail trash bin

In the Trash bin you can empty one at a time by clicking it then click delete on the toolbar or right click it then click delete.

Or all at once... from the menu on top choose Edit...Select All...again from the menu choose Edit...Delete

Also set the Mailbox Behaviour in Mail Preferences/Trash Mailbox to empty the trash for you at different specified times.

Extra info...Delete emails in Mail on Mac - Apple Support

More info...Change Mailbox Behaviors preferences in Mail on Mac - Apple Support