Welcome! This forum is a great place to learn, so buckle on your reading glasses!
Honestly, it takes years to grasp the development of Epicor. There are a ton of nuances that simply take working with it to understand. So be ready for some frustration as you work through it all. You’ll feel like you’re totally in the dark for while. It’s normal.
That being said, understanding what places customization and what the differences are is the first place.
Server side code is things like BPM’s (business process management) and Epicor Functions. This is where you can use C# code to do things. The basic IDE in there has some intellisense, but you have to use Ctrl+Space to use it. After a while that can become so automatic, you don’t even think about it.
Client side code used to be C# within customizations. These will be refereed to “Classic” customizations around here. Depending on where your company is in the kinetic journey, you may or may not deal with those anymore. You mention application studio, so you are probably working with the new browser screens.
Application studio is “Low code/No code” (which really means to me just some other coding syntax that causes carpal tunnel…). One of our users, Hannah has compiled an extremely long How To for all of the different controls.
Learn how to do a trace before you start any real code work. There are some posts around here to show you how to do that, but it’s a good way to see what epicor is doing natively and then you can use that information to recreate that in code, then sprinkle in the things that you need to do to automate things.
That’s some really super basic info, but over time you’ll start learning the normal patterns that Epicor uses to do this with the business objects, but there isn’t really a magic bullet to get you up to speed on it. You’ll have to work through them to see what they are doing.