The newer low-end models have various changes from the previous high-end models, including case changes, but also other hardware. The original low-end iPhone SE had the plastic case with the hardware of the 5-something - but a smaller display. The iPad mini had the guts of an iPad 2, except it was a smaller device, requiring a new motherboard and display, and it had a better camera, too.

Similarly, the 2017 iPad was closest to the iPad Air 2, just a bit thicker with more battery, but it also had a new CPU and brighter display.

That pattern of component upgrades and size changes continues, and as the GP mentioned, Pencil 2 support (magnets and charging) hasn't trickled down to any non-pro devices yet, in spite of new releases to those non-pro devices.