Insert a Video in an Device Frame

Possibly, though I would normally use After Effects for this.

If the device frame doesn't move or doesn't move very much, you could probably do it.

1 - Place the two files into a sequence with the device video on the track above the screen video.

2 - Click on the device video and open Effect Controls.

3 - Locate the Opacity Effect group and open it to reveal Opacity.

4 - Use the Free Draw Bezier mask button (the pen head) to trace the screen.

5 - Set the Mask to invert to cut out  a hole. You should then be able to see through the device screen.

6 - Then all you would have to do is adjust the size and position of the bottom video. Also done using the Effect Controls panel. If you need to distort the video so it matches the perspective of the device you can apply the Corner Pin effect from the Effects panel.

That might work. But if the video of the device moves you would have to animate the screen to move with it. And if it moves to much or changes perspective doing this in AE using motion tracking in MochaAE would be a better solution.

One way:

Get a photo of a smart phone and use an opacity mask to allow the video on a lower track to be seen behind it.

Adjust the size and position and/or use the Crop effect on the video.  If looking at the smart phone on an angle, add the Basic 3D or Transform Effect to adjust the video at the same angle.

You can also do it with a Track Matte.