You are confusing SAR (screen aspect ratio) with PAR (pixel aspect ratio).
Photography used 24 x 36 as the standard dimension for silver-oxide photo's and that has a SAR of 3:2. But in photography the PAR is always 1. That means square pixels.
Video used to have non-square pixels in the SD time, where SD NTSC has 720 x 480 pixels, but that can be both 4:3 format and 16:9 format by using different PAR's. This also applies to 1440 x 1080 recording formats that do not use square pixels but rectangular pixels, that are often converted to 1920 x 1080 square pixels on playback.
Google for the differences between SAR and PAR and you will learn about the differences.
Yes I am accustom to square pixels. I understand that aspect ratio deals with stretching them.
So what you are saying is that the number of pixels can be stretched to fit any aspect ratio (format)?
So it is like resampling in PS to get elongated pixels?
Here is what I don't get. Why does PS resample at a different aspect ratio than PR? Or is there something I am not getting?
3:2 vs 4:3 changes the size. What is the correct size?
That depends on your footage, but you need not worry, because if you use sequence settings that match your footage, you are OK for editing.
Photoshop uses square pixels, so the default NTSC resolution is 720x480, a ratio of 3:2. Premiere uses non-square pixels, making the default NTSC resolution 640x480 (for full-screen images), a ratio of 4:3.
The correct aspect ratio depends on your source material, always.
For Image production in PS, I will use the included New Image Presets, say NTSC 720 x 480 Widescreen. PS will be displaying square pixels, even though the PAR of the Image will be PAR = 1.2121, and will warn you of this.
For the New Image, just use the drop-down, and select the appropriate Preset.