I came across this video and I don't understand how a vacuum sealer works on a sealed glass jar. In the video, you can see the glass jars are closed tightly with a lid. In this case, how does the air inside the jar get sucked out to create a vacuum?
Not checked out the video, but if you put a jar with a liftable lid in a vacuum sealer then as the pressure outside the jar is reduced air in the jar will push past the seal to equalize the pressure.
Once the sealer is opened then as long as the jar seal is good the reduced pressure will stay in the jar.
What they are doing in the video is heating up the liquid inside the vessel, (with an open lid).
Then they take it out and immediately (i.e. before it cools down), they pack the lid.
What happens is that as the liquid cools down, and the temperature (in Kelvin) drops then the pressure drops. An approximation is the ideal gas law (although as Abel pointed in the comments, the pressure drop is even greater).
$$P\cdot V =n \cdot R\cdot T$$
Because the pressure drop there is a pressure difference with the exterior of the jar. That pressure difference is responsible for the tight seal.
They heat the jars and take them out of the oven open the lid to let the pressure out and then close it.
Once the jar gets cooler the pressure inside that was occupied with hot expanded air settles and the air volume shrinks back causing it to create a small vacuum.
Many times early in the chill mornings I see my bottle of water left in the cup holder overnight in the car is crumpled because the air in it got cold and shrunk, creating a vacuum that was crushed by the outside air pressure.