Please follow these steps to create a smart folder in iOS version.
Tap on the "Search" icon appearing at the top of the email list.

  Select the "Purple colored folder" appear in place of Search icon.

Please enter the search operators in the "Criteria" box of pop up window appear after selecting the folder. You can use the pre-defined operators which are already indicated in the smart folder box as shown in the picture below. These operators should be sufficient for almost all the quick searches. If you still need to define extra operators for the smart folder, you can use the Google Advanced Search Operators                                               https://support.google.com/mail/answer/7190?hl=en

How to include the Smart Folders in the sidebar?

  1. Please open the side bar by selecting three dash icon from the top left side of Inbox list and then select "Edit" from bottom.

        2. Enable "Smart folders" in the next list and then select Done from bottom.

           3. The Smart folders will appear in the side bar with other folders.

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