Deleting photos from Lightroom Mobile

I always upload my photos to Lightroom, on the computer (is that classic, i forget) , then add this as a collection.  If I need to edit while I'm away I then sync the collection folder to Lightroom mobile and edit the photos on my iPad, then re sync once I'm back at my desk.   I turn off the syncs to the folders, so that I only have a couple on LR mobile at any one time but my All Photos in the app on my iPad shows 11500 photos. Does it keep a copy of every photo ever added? I'm a bit confused!  I'm assuming I can delete them all as the photos are actually all saved and backed up on my computer.  So, can I delete these and why are they there?  LR blows my mind so I need simple answers please 🙂

Having just looked at the photos, there also seems to be hundreds from my iPad too, if i delete them will the be removed from the iPad library too or just LR mobile?  Thank you so much 😃😃😃