I don’t know how far in the weeds you’ve gone on this, but in terms of tech, a side lit TV is exactly what it sounds like, the “bulbs” are on the side of the tv, essentially illuminating the entire panel at once, and then the LEDs in the screen either let that light through, or don’t, and that’s how the picture is made. Some regular LEDs may have individual back lit zones I think, as opposed to side lit, but if they do they are large, like measured in inches.

Mini LED has a bunch of individual lights (LEDs) all along the actual back of the screen itself. Thousands, or even tens of thousands, of individual lights. Then the screen itself is still deciding how much of that light comes through and what color, but the light itself is coming from tiny individual sources. This makes them able to have very high contrast, one because the individual lights are able to be very bright without impacting any other part of the screen, and two because if you want dark, you simply turn off that zone completely, as opposed to trying to block light from coming through. There’s no light to block. Lastly, and this is just my own personal feel, I think your eye is able to perceive the difference between something that is coming directly from a light source, as opposed to reflected light. (This is where OLED really shines because on a OLED each individual pixel is itself a light source). On a mini LED those backlights are pointed right at your eye. I think this makes the picture feel more realistic. When watching a football game, it feels more like sitting in the stands looking at an actual game, as opposed to watching a picture on a screen. But that’s just me.