These two things are tiny, but they are very helpful. If you have some photos that you don't want to transmit everywhere, but still want to keep them around, you can hide them. The interface is really hidden (no pun intended, but here is what you need to do)
iOS Devices you Share to Hide
Ok, I actually think the naming of Share is pretty confusing, but you basically click on the share button with any photo in iOS Photos and then choose "Hide". I know sharing to hide is a bit weird.
Then to unhide, you have to go to
Utility/Hidden and then again choose Share and the Unhide. Share should really be called More I think.
MacOS you need to enable hiding
For some reason this is not a default option and it isn't really clear how to turn it on with
View/Show Hidden Album and then if you right click on any photo, you can hide it.
This was really easy before, but then in some version of Photos, they hid how to do it. The trick it with any photo, go to Get Info and then buried in the attributes is
Faces click on the plus and then you get a circle where you can name people. Geez, that's not so easy either.