Have to agree with HVAC-Notice, that learning how to use a software package like HAP or Trace is not the same as learning heat load calculations.
You need to have a fundamental knowledge of how to calculate heating/cooling loads before you dig into learning the software.
There are a number text books available, search for Heating, Ventilating, and Air Conditioning design and analysis on Amazon.
Carrier has published books in the past that are an excellent resource.
I'd hesitate to rely on any particular website or YouTube video. They tend to try to compress information into a restricted format and details get left out.
If you're wanting to get into HVAC design as a career, try to take a class in college/uni. Then find an engineer to work for that is willing to teach you what classes and books don't.