The structure of the video is simply a “flipbook”.
What is used in each page of the “flipbook” is an “image.”


For example, with a resolution of 1920 x 1080 (full high-definition) and “30 frames/sec”, 1800 images are required in 1 minute.
If you made this into a video as is, it would be a huge file size of 15GB in 3 minutes.


Therefore, if you want to save it as an uncompressed file (extension AVI, etc.), it is common to use a resolution of around 640 x 480 (300,000 pixels).(Even so, the capacity will be about 1MB in 1 second.)


If you cannot lower the resolution, you must compress each image one by one using a compression algorithm called a codec to reduce the file size.
Currently, the commonly used “codecs” are H.264 and VP9.
The extension of video files compressed with H.264 is MP4, and the extension of video files compressed with VP9 is webm.


By the way, H.264 includes patents, so a license fee will be charged.
For this reason, there are very few free software that has a video recording function that can save videos as MP4 files.
(There seems to be a lot of uncompressed AVI.)