Kodi: A standalone multimedia player, it plays contents stored locally or on the network (SMB, NFS)
Plex/Jellyfin: Multimedia server, it shares local content over network/internet to multiple users.
Stremio: Plays content from online sources (torrent, HTTP)
I tried to keep things simple, however I use Kodi mainly with PlexMod addon.
Plex official apps have many issues with audio and subtitles, Kodi plays (almost) everything and better options for audio/subtitles