I am trying to develop and understanding of these two apps…
I am guessing that when Samsung bought ST they had to put their own stamp on it which is why the new app was created.
Will things (rooms, lights, automations, whatnot) created in one be able to be managed in the other? In other words if I start using classic and it disappears or ceases to be supported, will I have to reconfigure things to continue using the ‘new’ app?
Good guess, but no. Samsung bought SmartThings about four years ago and has put out multiple app versions since then. The new app is part of their 2020 strategy to have all of their smart devices, including their appliances and televisions, work through one unified app. But since it does not yet support all the features of the Classic app (that’s a separate long story), they’re keeping the classic app around for a while yet.
It varies from feature to feature. Start with the following FAQ, read it, and then follow the links in it to the other informational resources about the new app and that should answer most of your questions. 
( This is a clickable link)