I’ve been using PIC
for years these little IC’s are very useful to build both basic
and complex circuits with.
They basically function like a mini computer they have a CPU consisting of registers and an Arithmetic logic unit data and address buses program storage space and RAM plus IO to communicate with the world, because what’s the point in having such a device if we cant interface with it in anyway.
See below for a block diagram of a typical PIC micro controller.
The beauty of these IC's is they replace complex board layouts that originally would need many other discreat logic IC'c like the 74-series.