One item to consider when setting up a Plex server for the first time is space allocation. Plex servers can use up a lot of disk space and not necessarily where you expect, so it’s better to be educated than blind sided.
The vast majority of space you can expect to use on your server is going to be for movie storage. The location where you store your movies is completely up to you and is configurable when you first set up your libraries. In my case, I created a path called /home/media on the server which contains directories for movies, tv shows, music, etc. Each of those directories results in its own Library.
The less expected space is under your /var partition, which can be a problem if you allocated a small amount of space for / or /var and put the majority of your disk space under /home. (As an aside, consider using LVM or ZFS for your disks in order to be able to reallocate space on the fly. I’ll have to write up more about this in the future).
The space in /var will be found in /var/lib/plexmediaserver/Library/Plex Media Server on Ubuntu. I suspect other Linux distributions will be the same, although there could be some variance if you’re using something like FreeNAS or installing Plex via a container.
This space contains a lot of really useful things, for example your installed plugins, logs, the metadata for your server, and transcoded media files. The last one is where the bulk of your space will go to. On my server this space in /var takes about 20GB (compared to 2TB in /home/media). Plex does however support preemptively optimizing video files for playback. This feature is useful on slower hardware that may not be able to transcode on the fly, but the resulting media files will also be stored in /var.