Couple of noob questions about Plexamp

After doing a little more Googling regarding music player apps I ran across an app on the Apple store called Prologue (for Plex) - it cost me a one-time fee of $5 or $6 to activate the premium version so I could download my files from my Plex server, but it seems to integrate with my Plex server very nicely and lets me play my audiobooks (or any mp3 file apparently) offline on my iPhone without having to be connected to my Plex server, and it keeps track of (at least the last 3) files I have been listening to and lets me resume listening from wherever I stopped listening to them. I’ve only played around with it for a few minutes and haven’t tried it in the car yet, but it does say it can throw the books onto the TVs via Apple’s AirPlay and it also supports CarPlay, so I’m hoping I can get it to bluetooth to my car’s speakers. I was hoping I could use Plexamp for this, since it looked pretty good at first on my phone, but failing that I guess this Prologue thing will be a good solution.