don't quote me on this but i think kodi/xbmc started back before smartphones weren't really a thing. people mostly had 1 desktop computer and they would watch movies on their monitor or with the computer plugged into the TV.

plex was made later on when having multiple devices (desktop, laptop, phone, tablet etc) was a bit more common, so now instead of having all your media stuck on one device you instead run it on a server and connect to that server using an app on each device.

so that's the fundamental difference between them anyway. one is mainly design to be used on 1 devices while the other is design to be used on many