The choice regarding unlimited Google Photos storage is in
Google Photos > 'Hamburger' menu (top left) > Backup & sync > Upload size.
High quality free unlimited storage re-compress photos and videos to reduce the file size, but you will probably not notice any difference in picture quality.
The 15GB limit is for original sized photos only, if you select high quality it is unlimited but the images are reduced in resolution if they are over 20MP as well as recompressed.
I have just short of 87,000 images on it which on my HDD takes up over 340GB but my Google Drive space is unaffected.
Yes and the second post in this thread explains all you need to know.