The AR pattern of rifles has also evolved into something highly modular in the past 20 years. The same lower receiver can be swapped into an upper receiver set

The AR pattern of rifles has also evolved into something highly modular in the past 20 years. The same lower receiver can be swapped into an upper receiver set up for anything from long distance precision target shooting to close-quarters room clearing. Saying someone owns an AR is sort of like (loosely) saying they own a PC - that could mean anything from an Intel NUC to a Xeon workstation in a huge case. There's a common architecture, but it's the flexibility of the architecture that makes it popular.