How do I delete all of the emails in my junk folder at one time?

Check your mail account. Some accounts use Archive, as it puts the messages into another folder and does not delete them. You have the option in the account to set that as Trash or Archive. Go to Settings>Passwords & Accounts, select the mail account in question, then tap on the Account again, then down to Advanced, then select Deleted Mailbox instead of Archive Mailbox.

Hello,

In Mail tap Select > Select All > Delete.

Hello,

With email providers like Gmail, they have decided to rename "delete" to "archive". It serves the same function. I am not sure why this decision was made.