TIL you can enter a photo description in Google Photos. Select a photo, click the Info icon, and a free-text “description” field is revealed. Not super sure what the description is to be used for, but I’m hoping to use it for photo captions.
Oh, but it looks like you can’t get the description as an EXIF tag when you download the image. That kinda sucks actually. Might be possible with Google Takeout (based on reading a passing comment in a forum).