Welcome!
Back in the beginning when smartthings started, The official features didn’t have very many options, so people would write their own smart app for the specific feature they wanted, and then submit it, and quite a few of them got published for other people to use. It’s also possible to just share your code informally, and we’ll get to that in a minute.
Meanwhile, time went on, and two things happened.
First, the official features were expanded considerably, most notably with the addition of the official “smart lighting” feature, which can do a lot more than just lighting. And the addition of smart Home Monitor, which can do a lot with sensors. So once both of those were available, the need for a little tiny individual smart apps that just did one thing was greatly reduced.
Second, for those with a little more technical background, community members created a very sophisticated rules engine called Core, which eventually evolved into web core. This is essentially a scripting language for SmartThings and can do pretty much anything you can think of in terms of rules. Very powerful. Also, pretty complex, although there are many community members will be glad to help you with it. So if you are comfortable with that kind of approach, definitely go straight to web core. 
There is a really hacky workaround which would allow you to assign multiple contact sensors to one zone and then use the zone representation in another smart app but to be honest at this point that’s going to be way more work than just starting with web core.
So now you get your first, and probably most important, lesson about SmartThings: it’s very powerful, it’s not very intuitive, but there will always be lots of community members who will be glad to help you find a way to do almost anything.