If you haven’t already, check out the “YAML for non-programmers” link in the Community Cookbook, it’s a good primer for some of the basics of YAML structure.
FWIW, when I started with HA (with zero YAML knowledge) I found the different methods shown in the “Splitting up the configuration” article very confusing. I think I had read it 3 or 4 times before I read the section that mentions Packages and actually clicked through to the Packages docs. For me, using Packages just made way more sense than all the other merging options.