Your camera is operating as it should. "Liveview" is a term that describes using the rear LCD as a viewfinder.

In the "OLD" days, film cameras used some kind of an optical viewfinder.

When digital cameras came long 35mm Digital SLR's continued to use an optical viewfinder while "Point and Shoot" compact digital cameras and "Mirrorless" cameras tended to use the rear LCD display as a viewfinder.

When this feature was added to DSLR's it was called "Live View".

"Exposure Simulation" can show you what the exposure of the photo will look like on the rear LCD, before the photo is taken. It can be turned on or off on some cameras, while some cameras will always default to a full brightness display of Liveview. There can be advantages to both types of Liveview displays, so it is nice to have the option to turn Exp Sim on or off. Your M50 has this option.

You need to learn about the EXPOSURE TRIANGLE. Aperture, shutter speed, and ISO all interact to give you the correct exposure. If you change one, one of the others must also be changed to retain the correct exposure.

In your video the photo brightness does not change because you have Auto ISO enabled which automatically changes the sensor sensitivity to compensate for the changes you made to the aperture.

If you want to see the effects of any changes to shutter speed and aperture on your photo, you need to shoot in Manual mode with manual control over aperture and shutter speed. You also need to use Manual ISO and set your own ISO. THEN, if you have Exposure Simulation enabled, you will be able to see the brightness effects of any changes you make to any of the THREE parts of the exposure triangle. (Aperture, Shutter Speed, ISO)

The purple sticker in the background became sharper with a smaller(higher number) aperture because of "Depth of Field". You can think of Depth of Field as the Depth of Focus. DOF increases with small apertures and more of the background and foreground will be in focus as you use smaller apertures.

Some cameras have an option to enable "Depth of Field Preview". You should be able to enable Depth of Field preview in your M50 using "Configuring Custom Functions" or "Assigning Functions to Buttons".

From the M50 user guide: