Once you've added a photo placeholder, tap its photo icon and choose the option "My Files" with the purple Creative Cloud icon.
This lists various Creative Cloud assets. There's no Lightroom icon, but you get to Lightroom Mobile by choosing Photos.
All the collections synced to Lightroom Mobile are listed. Choose the one you want.
Choose your photo from the synced collection
Comp adds it in the placeholder
When you're finished, you can send the composition to Photoshop, InDesign or Ilustrator
A minute or two later, it automatically appears in Photohop
The Photoshop file is easy to edit because the images are smart objects and the masks are applied as vectors