Why your sens changes in 4:3 stretched
In 4:3 stretched, the game renders a narrower 4:3 image and widens it to fill your 16:9 screen. Everything near the crosshair is stretched horizontally: the same mouse movement moves the crosshair 33% further on screen. Models look wider, and your sens feels faster horizontally.
To keep the same feel near the crosshair, multiply your sens by 0.75 in CS2. The 360° (full turn) doesn't change: it's the fine aim that does.
What about black bars?
With 4:3 black bars the image isn't widened, so the feel at the center stays the same as 16:9. You just see less on the sides.
Games change their FOV with the ratio
CS2 and Apex keep the same vertical view and cut the sides in 4:3 ("Hor+" FOV). Valorant always forces 16:9, so 4:3 stretched doesn't really exist there. The tool accounts for how each game computes its FOV.