A lot of airports are privately or quasi privately (Canada has a non profit called NavCanada) run which allows for advance planning of CapEx instead of depending on Congress. The US system where the FAA oversees and runs ATC is, to put it nicely, unique.

I don't think so. I believe the UK had their own, but NATS chose to switch in about 2002, and finished transition in 2007.

I think truth is more so that each is using contemporary technology from time the project started. So those that started later have newer. But that does not mean modern. As in current day.