Have you tried to use the project Generator for adding addons to your project ?
You have to keep in mind something. The people who created openFrameworks did their best to make something that used to be pretty complicated, become easy to use for non-specialist, but underlying are still pretty complicated thing, so if you want to understand what is hidden behind what oF does for you, it’s a fairly complicated journey (depending on what’s your background I guess, but can help you learn much about other domains)
Your question kind of lack of details : you’re not saying what IDE you are using (trust me, using Visual Studio is probably bitchier than XCode, but from your other posts you seem to be using the latter), nor what video tutorial you’re mentioning. I think if you want to be more constructive, you should explain in what adding an addon is a tricky process, or even better, explain in your own words how you had to proceed, so it might be helpful for other people in your situation. People more seasoned tend to forget some of the complicated steps they have to go through as it becomes natural to them, so a fresh point of view might be useful for further explanation.
On the other hand, for yourself, you should probably try to have a horizontal approach to your problem : if you have no idea on how to use XCode, don’t know anything about C++, maybe you should look for resources that explains the basics of how to use the tool you want to use (it’s like if you were asking why it is so complicated to use oil paint in a course about nature morte… If you’re lucky someone will tell you the basics of oil painting, but maybe you could research the subject as well)
And in a nutshell : every .cpp file you have are going to be compiled as object files, .h files are meant to tell other objects “this guy (other object) over there can do that and that and that,… , so if you need this function, you’ll know it exists and who to look for”, and when everything is compiled as objects, a second pass called the linker is going to to make the link between all the function in all the object, and pack everything in a binary that your computer can run. But for all this to work, you need to tell the IDE the folders in which the files are, the libraries (big chunk of objects) you’re going to use, and so on. How you tell your IDE depends on which IDE you’re using.
As mentioned before, the project generator is here to ease the process. And I’m sure if you have ideas to improve the whole process, suggestion or tutorials could be helpful to everyone