Songwritting musically is a craft. A songwriter is like a carpenter building a house. The more tools you have, the more things you can do. That's it.

When you build a house, you have to start with a solid foundation. Same with music, one solid idea will build the rest of the song. Understanding the different aspects of music, dynamics, rhythm, melody, etc. Forms and how apply them is all part of the craft. And there are several tricks to doing so, and each one of these tricks is a different tool.

Those who say they write something and don't know how to combine it with something else don't have any tools. Listen to your favorite songs and think about how they got it to work, the tools are all in listening.