There are three dots to the bottom that read Cast to device, zoom to fill, repeat, autoplay, play as 360 degree (uses a symbol not on my keyboard) video. There's an arrow going right which will play the next video. There's a conversation icon which allows for subtitles, choose a subtitle file, Audio, and a checkmark for language type. On the bottom there's a forward arrow which will allow the current video to play and a 30 with an arrow in a crescent shape atop. You can move to the next video and play it. You can play the current video. Since I chose the top video in the folder there is no video or left arrow going back. However there is no exit form this playing mode once inside. I don't want to believe Microsoft programmed Windows 10 Mobile this way but it seems the only way to back out and go back to the directory tree, or anywhere else on the phone is to power cycle the device and go back to the menu screen.
I'm going to leave it right where it's at until tomorrow when I get the SIM card and service for this phone and have someone at T-Mobile back out of this video viewer screen for me and post the solution here so it can help the next person behind me. If I don't power cycle the device the next time I wake it up it is right back in the video play mode.