As many don't know about it, we can't delete mass mails on a Gmail mobile application instead of the desktop browser. But you can select/tap each mail to select all and delete it. But this is not the right way to delete the bulk of thousands of emails.
Let's check out the step on how to delete bulk mail with the desktop version.
Open Gmail on your browser window.
Use the search bar or select a category.
Search: older_than:1y, label: promotions, or is: read
or click on the "Primary", "Updates", "Social", "Promotions", etc.
Click on the checkbox at the left to select or unselect the emails.
Finally, click on the delete button to remove all emails.
Note: After deleting the emails from the categories, make sure to permanently remove them from the trash folder.
Pro Tip: To never want those unwanted emails back from the same sender into your mailbox, then unsubscribe from promotional emails or newsletters.