How to capture still frame from video on iOS?

I highly recommend a free app called Frame Grabber. It captures still frames from a video, maintaining both original quality up to 4K and metadata (datetime, gps...). It's free and I find it brilliant.

No, there is no option in the photos app to extract a single frame.

Depending on the video resolution you may or may not end up with a lower resolution screenshot. If your video resolution is higher than the screenshots' resolution, you end up with a lower resolution image in the end. Read more about video resolution on iPhones and screenshot sizes.

The easiest way though would be to either use a third party app or - if you import the videos to a computer anyway - do it on a Mac/PC.

More about this: different methods, Apple Community