how do I view the trash and junk mailboxes

They are not added to Favorites automatically.

They will be listed under each account in the Mailbox list.

If you want them in the Favorites, hover cursor over Favorites header and click the + Button.

Add what you want from that popup menu.

Are you saying that those mailboxes are not in the sidebar of the Mail app?

If not present then move your cursor of the heading (e.g. On My Mac) where you want to add a mailbox and a + sign will appear just right to the heading. Click on the + sign and a window opens that has a dropdown items then select which mailbox you want to add and select it. Click on OK to add it.