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

You can get into this mode by long tapping an email in the email list view or by using the Vertical 3-dot menu | Multi-Edit.

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

The actions include Mark all, Delete, Move, Archive, Mark as Read / Unread, Mark as Done / Undone, Star / Unstar, and other depending on context and email provider.