You can perform an action on multiple emails at once by using Multi Edit mode.

You can enter this mode by long tapping an email in the email list view or by using the top bar vertical 3-dot menu | Multi Edit.

Once in this mode, you can select the emails you want to apply an action to, and then perform the selected action on all of them at once.

The actions available are Mark all, Delete, Move, Archive, Mark as Read / Unread, and Star / Unstar.