Everything imported into Lightroom Mobile will sync to Lightroom Desktop.
If you are going to use the native camera and not the Lightroom one, then there are only two options to import from the Camera Roll, manually from the app or setting Auro Import, although this imports everything.
Usiing the other apps from photos that reside in Lightroom would require you probably use the Open In command or Share to. The current state of the edited file would be exported and when brought back into Lightroom from the other apps, all your previous Lightroom edits would no longer be non-destructive.
If I set LR Mobile to Auto Import, that means I'm essentially creating a duplicate of my Camera Roll in iOS in the LR cloud?
For the past several years, I shot with a Fujifilm X-T3 and organized/edited in LR Classic. The vast majority of my photos are organized into collections here. Most recently, I've been shooting with the Camera app and editing in Darkroom or Snapseed. I now have several albums in Photos.
Now I'm considering switching to Lightroom Mobile on desktop and using Lightroom CC (I know it isn't called that but just trying to be clear) as my primary editing app on the iPad and iPhone so that everything is in one place and I'm getting the benefits of the LR ecosystem. I will still use LR Classic to print, but otherwise it seems like a combination of LR CC (desktop) and LR Mobile might meet my needs. I guess I need to commit to one process or another; otherwise I'll end up with photos in too many different places.