This is my first time posting here and I'm sorry if any of my questions are silly. This is my first time dealing with open source code and licenses.
I'm working on a small game engine and I decided to use this person's tutorial as my starting point. His engine is available on GitHub under the Apache 2.0 license.
I, too, decided to put my engine under the same license, but I don't have any intentions of distributing my engine/ code in any way. I am building this engine (which I may not even finish) purely for educational and self enrichment purposes, but since I will have the code on GitHub I know that anyone can access it.
I didn't exactly fork this person's engine over to my project, what I'm doing is following along with his tutorial from day one starting from the ground up and reimplementing his ideas into my own using methods and design patterns that I'm learning/ want to explore.
On occasion, I do take some of his code to use if, for example, I don't have an interest in learning how to program a particular subsystem or if at the moment I don't have an idea for how I would reimplement something as my own.
How would I go about applying the license to these individual code files that I'm taking from the developer? If they are in a folder is it enough to include the license in the folder or do I have to include the license in each file?
Additionally, I understand that I must include a report with all the changes done to files I modify? Where would this file go and what constitutes modification vs. my own work. Say, for example, I use someone's algorithm under the Apache 2.0 license but rewrite about 85% of it to work for my use case and keep 15% of it original. At what point would something count as being original rather than modified?