Videos for the web should have an aspect ratio of 16:9. Videos with an aspect ratio of 4:3 will produce black bars.
For websites best size is 1280 x 720 pixels with a lower frame. Anything higher could mean the video will be slower to open on lower specification devices and low Internet connection speeds.
For Social Media platforms you can use 1920 x 1080 which is an aspect ratio of 16:9.
Videos for viewing on a modern TV should have a minimum resolution of 1920 x 1080 pixels.
The first example shows what happens when the incorrect aspect ratios is used - black bars will be produced. The second example is correct.
Starting the second video while the first is playing will paused the first. To restart the first video click / tap it. It will resume play while automatically pausing the second video.