That’s up to you how to organize it, just do whatever thing most logical for you. I myself usually went with this setup:
mounted a dedicated SSD partition for docker /var/lib/docker path. That docker directory tend to grow larger and larger over time so I don’t want it to suddenly fill up my root partition when I’m not looking. You may not need to do this if you plan to create just a handful of containers in your machine.
mounted data partitions in /var/lib/host-path. Depending on what kind of disks available in the host, I may have an SSD or HDD partition mounted inside that directory (or both, or not at all so it’ll use the root partition).
container data directory live inside the data partitions, e.g. /var/lib/host-path/HDD/Plex. If the container requires multiple volumes, just put them as a sub directory there, e.g. /var/lib/host-path/HDD/Plex/Movies, /var/lib/host-path/HDD/Plex/configs etc.