Guys, I know I am going to come across as a dick. . .
Have any of you asking the question over and over compared the god damn photos and specs posted. This isn't fucking rocket science.
Magwells and contours all over are different. The uppers are interchangeable. One mag-well holds less of the mag, but fits other (PX9 vs PA-X9) both are glock style.
The Pa9 uses COLT STYLE MAGS And is a FORGED lower. Similar to a regular standard AR15 lower receiver.
The PX9 uses GLOCK mags. It has some contours on it that don't match up correctly unless you use a PX9 specific upper receiver. ANY of the PSA 9mm uppers will work on it but the lines WILL NOT MATCH UP.
if you want to use another companies upper like RRA and you want the gun's machining and lines match, you want to use a PA9 lower with colt mags.
There is also a PA-X9 that uses glock mags BUT fits regular RRA/COLT 9mm uppers better and looks correct. This lower wasn't mentioned in the original question but is worth bringing up.
I own a minimum of one each of the upper listed receivers and can attest to their fitment.