The main difference is the how/where the media is consumed. With Kodi, the primary consumption is at the place where Kodi runs and typically there is no server

The main difference is the how/where the media is consumed. With Kodi, the primary consumption is at the place where Kodi runs and typically there is no server. kodi can support lots of plugins and act as a interface for different media sources/apps. This makes kodi feel a bit clunky. With Jellyfin & plex the primary place of consumption is a client running on your favourite device while the heavy lifting of downloading and transcoding is done on the server. This abstracts the different media sources from the clients and clients are just focused on consuming media.