Even as a full time mac user, both at work and privately I have a hard time remembering which release, say Sierra was and which one Mojave was and which one came first and if any release was in between.
Strangely I think it was easier to keep track of the order of OS X codenames when they were all big cats. It was more memorable.
Vista was the first of the NT 6 series. However Windows 7, 8/8.1 and the related server versions were also NT 6.x