Hello friends! There are a lot of photo and video editors. But as for me, I'm used to using the best video editing platform, which includes a lot of features, including creating thumbnails for videos, which you can read in more detail here https://wave.video/blog/video-thumbnails/ Indeed, for creating photo and video content , you need to choose really high-quality services, because the success of your project depends on it.
I do not know what about photo editing and exactly on the iPhone. I know that there is a program for Mac screen recording (https://www.movavi.com/screen-recorder-mac/ ) well I think if you go to their website there should be programs to work with video and for IOS. If you do not find it, then write me a personal message.
You can also consider LightX (android, not sure if iPhone has it), or Adobe Lightroom if you have Adobe subscription :)
If you have an iphone, try downloading Enlight.