Although the iPhone has been able to take still photos while recording videos for a while now, you have to think about it at the time. The good news, though, is that if you’ve ever found yourself wanting to grab a still image from one of your iPhone videos after the fact, it’s actually pretty easy to do this using the macOS Photos app.
You’ll have to transfer the video to your Mac first, of course, but if you’re using iCloud Photo Library, it should automatically be there. Find the video and go into playback mode and you’ll see a setting icon on the very right hand side of the playback controls bar, on while you’ll find the option to “Export Frame to Pictures.” Simply play back the video, or scrub to the current frame that you want to capture, and then select this option; the frame you’re currently viewing will be saved into your Mac’s Pictures folder as a TIFF image, named “Frame” followed the date and time that you exported it. This doesn’t save the still image into your Photos app, however you can import it from your Pictures folder easily enough if you want to keep it in your library.