Meet was originally enterprise focused, with features like being able to record video, tight calendar integration, etc. Chat is also enterprise focused, similar to Slack.

Duo is targeted at consumers, and has e2e encryption among other things. It similar to Apple's FaceTime.

Hangouts is old school Google+ tech, the whereas the others use newer tech. E.g. There is some impressive tech behind Duo: https://ai.googleblog.com/2020/04/improving-audio-quality-in..., https://www.blog.google/products/duo/4-new-google-duo-featur....