Conventional Video
Video files can be used in AR in different ways:
Floating video screen which can "play" a video in a location of your choosing in your space.
Video files can be inserted into a Portal in Hoverlay.
One of the first issues to overcome is the file size. The personal (free) level limits file uploads to 20 Megabytes, and many videos taken with your cellphone will exceed that even in a small (short) clip. To reduce the file size you can either:
Trim it by shorten by removing a segment from the beginning or at the end of your clip
Some cellphone camera apps have built-in "trim" in their tools.
Crop it by using a video editing software on your phone or on your computer.
Mac (Apple) computers have a free video editing tool called iMovie
Change the resolution using transcoding software
Handbrake is a free package that will convert (transcode) your clip to lower resolution (and smaller file size) - there are some key parameters to watch for, in particular "bitrate" (how many bits per second) that determine the size of your finished video file. https://handbrake.fr/