There is a new feature on Google Photos named Animation, you can use this feature to create an animation image with gif format which is created from several images or photo, the number of allowed images or photos is between 2-50 images. First, to use this feature go to photos.google.com with your web browser then select Assistant and there's some options to Create New then select Animation.
On this occasion I tried an experiment, I tried this feature to create two animated image using the same source means the first or second animation using some of the same images, the same picture number is 20.
In the first experiment the number of photos is 20 the result is as below.
In the second experiment with the same photo source as the first experiment and with the same number of images the result is as shown below.
So I can conclude from it, that the two experimental results: First, the system combines the images sorted by the date and time of image capture. Second, seen from the results, the pictures taken are not full frame of source images that sometimes look like a random focus.