1) it doesn't work outside of iOS/macOS really. 2) it's an app on both platforms with it's own communication protocol

You can use Google Photos similarly and it's cross platform. Not sure about commments tho

Line and Facebook also have "private" social media feeds. By private I mean only people you approve can view. No idea how mined they are for data for ads

AFAIK, it's IOS only. The one android user in my family switched to iphone after feeling left out for a year. Other groups also are all iphone

The feed is actually in the Apple Photo app. There is a section for "shared albums" and a link to "activity" that has a feed UI.

I've always thought if the shared album was tied directly to the group message that would be nirvana, but I don't believe that's a possibility.