Part 1: i-number (i3, i5, i7, i9)
The first part of the intel CPU names is the i-number. This is what class they are meant for. i3 is more geared towards laptops, i5 is for budget pcs and higher class laptops (like older MacBooks before they switched to ARM processors), i7 is for higher class desktops, and i9 is for extreme gaming machines.
Part 2: Thousand number (9980, 5420, 7800)
The thousand number is basically the Gen number (10th gen, 5th gen, etc.) the i-number as the second digit, and then the SKU of the product as the last 2 digits. e.g: 10 (10th gen skylake), 9 (core i9 [technically part of the SKU but corresponds to the i-#]), 80 (SKU), XE (eXtreme Edition) to all make up the 10th gen Skylake i9-10980XE.
Part 3: letters (9980xe, 7700k, 5420kf)
The third part of these confusing CPU names is the letter(s). For example, the XE in the i9-9980XE, means eXtreme Edition, while the K in the i7-7700K means overclocKable. KF means overclocKable with no integrated graPHics, while the K has integrated graphics. H means High optimization on mobile. HK means High mobile performance, overclocKable. HQ means High mobile optimization, Quad-core. S is Special edition, while T is power-opTimized. U means mobile power-efficient, and Y is mobile extremelY low power.
I hope this post clears up a lot of confusion surrounding these cryptic CPU naming system. Thanks for reading and have a nice day!