The Samsung video editor app was not included in the phone by default. You can download it either from the Galaxy App Store or Google Play Store. There are also several other options for video editing apps you can try.
There are many Android apps for you to merge video on Samsung Galaxy S7.
1. Video Joiner
2. VideoShow
3.Video Collage (VideoWhiz)
4.Video Merger to Instagram
5. AndroVid Video Editor