If you double click a photo, or select one or more photos then hit the space bar, that will open a QuickLook panel that shows you the full size version of the selected photo(s). Note that the QuickLook panel has a button that lets you expand it to fill the screen, which can be useful for a quick, full screen slideshow of multiple photos.
If you want to reveal the actual photo file in the Finder, control-click on a photo, and select either “Show File” or “Show Original File”.
If you want to do something with a particular photo in Photos itself, control-click on the photo, and choose “Select in Photos”. That will open up that library in Photos and select the image in Photos’ main window, so you can then do whatever you want with it.