There are people who, as a hobby or sometimes even a sort of profession, look for old photo albums in thrift stores and similar. Sometimes, they will try to find the people who owned the pictures, or even their descendants, and do so successfully. There are also what are called "found photos" - u/turdferguson3891 smartly mentioned these in his comment - these collectors take the old photos and make up back stories for them. I've done a few myself, it's fun.

In terms of format, and others can disagree with me, when you have a photograph where you can't identify everyone, it's ok to do something like this - "Bob Jones, unknown, Adam Smith, Betty Page, unknown" and similar.