To add to the above, you will almost always see a slight difference between screens due to manufacturing tolerance, especially in side by side comparisons (but note this can also be influenced by not having comparable brightness levels). Apple do calibrate there screens to reduce this effect though. The biggest change I believe is that the latest iOS devices are now being released with a slightly warmer tone compared to there earlier generation devices, and I’m no expert but it might also have something to do with the newer screens wider colour gamut.
Larger differences though might be down to settings, for colour you have two locations, Settings/Display & Brightness, namely the True Tone toggle and Settings/General/Accessibility/Display Accommodations for Colour Filters, playing around with the latter, or if something has been set by accident may account for this larger difference.
I generally have True Tone off as well as no Filters, and when side by side my X definitely has a warmer tone when compared to iPad Mini 2, but photo’s displayed on the X’s screen just seem to pop and much are preferred.