Correct. This came from the ITU-R BT.601 standard, one of the first digital video standards authors of which chose to define digital video as a sampled analog signal. Analog video never had a concept of pixels and operated on lines instead. The rate at which you could sample it could be arbitrary, and affected only the horizontal resolution. The rate chosen by BT.601 was 13.5 MHz, which resulted in a 10/11 pixel aspect ratio for 4:3 NTSC video and 59/54 for 4:3 PAL.

>SD channels on cable TV systems are 528x480

I'm not actually sure about America, but here in Europe most digital cable and satellite SDTV is delivered as 720x576i 4:2:0 MPEG-2 Part 2. There are some outliers that use 544x576i, however.

My understanding is that televisions would mostly have square/rectangular pixels, while computer monitors often had circular pixels.

Or are you perhaps referring to pixel aspect ratios instead?

What you're referring to stems from the assumption made a long time ago by Microsoft to simplify the development of Windows, later adopted as a de facto standard by most computer software. The assumption was that the pixel density of every display is 96 pixels-per-inch [1].

As the pixel density of today's displays has grown much beyond that, mostly popularized by Apple's Retina, a solution was needed to accommodate legacy software written under this assumption. This resulted in the decoupling of "logical" pixels from "physical" pixels, with the logical resolution being most commonly defined as the "what the resolution of the display would be given its physical size and a PPI of 96" [2], and the physical resolution representing the real amount of pixels. The 100x100 and 200x200 values in your example are respectively the logical and physical resolutions of your screenshot.

Different software vendors refer to these "logical" pixels differently, but the most names you're going to encounter are points (Apple), density-independent pixels ("DPs", Google), and device-independent pixels ("DIPs", Microsoft).

[1]: https://learn.microsoft.com/en-us/archive/blogs/fontblog/whe...

[2]: https://developer.mozilla.org/en-US/docs/Web/API/Window/devi...

If a video file only stores a singular color value for each pixel, why does it care what shape the pixel is in when it's displayed? It would be filled in with the single color value regardless.