Retina Displays

I can't find a good spot to post about the Retina Display technology Apple uses, so the iPad forum seems as good as any. I have an iPad Pro 10.5" (MPDY2LL) with a Retina Display at 264 psi. My iPhone XR has a Liquid Retina HD Display at 326 ppi. The latest iPad has a Liquid Retina Display also at 264 pi. Those are Apple's marketing names. According to the Wikipedia article on Apple's Retina displays there are a lot more flavors of Retina displays. BTW, my 27" iMac with 5K Retina display only has 218 ppt, but it looks pretty darn good. So maybe pixel density isn't the final word on picture quality.

What is the difference in the various Retina displays?

While all the TV reviews claim OLED is better in some respects than LCD (all Retina displays are LCD) the price makes me think Apple will never push OLED in a big way, the iPhone 11 not withstanding.