15 inch TFTs are roughly the visible size of a 17" CRT. They have 1024 x 768 just like it is recommended for a 17" CTR.
17 and 19" TFT have 1280 x 1024 which, yes, is 5:4. Somehow, in their infinite wisdom, VESA decided to make that nonsense one of their standard modes.
(Probably was about making the most out of some typical limit in physical video memory.)
19" CRTs will take 1280 x 1024, but then, well, it will be compressed in height, as normal 4:3 would be 1280 x 960
There are 20" TFTs in 1600 x 1200, but relatively rare as those were high end and when they got cheaper were replaced by 16:10 widescreen ones like 1440 x 900.
Aspect ratio for Windows is unimportant apart from ergonomics. A windows screen can have any resolution native to the screen. That's why hardly anyone noticed the weird AR.
In general, with video coding, AR and resolution are not the same. Most DVDs are anamorphic, meaning a resolution with a pixel count other than the intended AR coded into it.
Almost all CRTs were 4:3 but the games were not necessarily.
Most DOS ones were in 13h which is 320 x 200.
Which would be an AR of 16:10.
But, they were intended for a 4:3 screen, at least most. Meaning, their graphics employed "non-square" pixels. They were supposed to get somewhat stretched.
Some actually look better if you manually reduce the CRT screen to 16:10 or 4:2.5 height:
- That's a screencap in 320 x 200 - stretch that to 4:3 and you'll get an egg shaped planet.
So, not an exact science 😉