To test this new feature immediately, beta users need to update WhatsApp version to 18.104.22.168+. And if you are lucky, you might be able to use this new advance search right away. WhatsApp is enabling this feature from the server side so it might take some time for activation on beta testers devices.
What is the Advanced Search Mode?
It’s a feature that allows users to search a specific type of message (text, image, video, GIF, audio, documents). It is implemented in the global search feature. This allows users to quickly see all types of shared files globally or in specific chat windows.