How do I know what photos have been downloaded to Photos on my iPhone? Does Apple continuously remove the copies from my local device after a period of time so that I don't run out of space? E.g., I have 80Gb iCloud storage taken up by Photos. I browse my photos through the iPhone Photos app and it downloads images that I linger on. I only have 12Gb space in total available on my iPhone, will I run out of space in the near future or are these downloads removed and reverted to thumbnails at some point?
Yes, your iPhone will remove originals that you have not browsed recently, and then re-download them when you next look at them. If you want to learn more about it, lookup a cache. With the new Photos app, the originals are stored in the cloud, and the images on your phone are just a cache. (Except that when you edit a photo in the cache, Apple preserves both the original and the edits in the cloud.)