Multipart archives still exist. They’re now used for file sharing websites that have a maximum file size. Before that they were for unreliable p2p networks, so you didn’t lose the parts you’d already downloaded when your peer goes offline. Originally it was to fit something big on multiple cd-roms or floppies.

Opening somthing.rar also reads the data in somthing.r01 through somthing.r15 etc

Most compression programs offer a way to separate your thing in multiple parts, I know 7zip and Peazip do.

I’ve recently had to properly rename the latter part of a multipart zip because the source I got it from probably just renamed the parts it stole from elsewhere, which broke the whole “extract part 1, everything else comes along!”