This has the same kind of infuriating quality that most cmake documentation does. It explains in detail how to do obvious stuff but has nothing to say about what the execution model behind the syntax is.

I mean, if you learn classic make, one of the very first things you learn is "this is a target; it's just a file; the stuff on the right are dependencies; the commands below will be run whenever the dependencies are newer than the target". Now, there are implications to learn, but that is the model, and it's obvious: make runs programs to create files when the stuff needed to create the file changes.

Now, how do I do that in cmake? It's almost not even here! It's all about C++ files and libraries and how to define variables and write functions and include other cmake files. But what is it going to do with that stuff? It's all just magic. Type it like this and it'll work. Cmake understands your compiler better than you do.

What docs there are on creating a custom target are buried in two paragraphs inside a subsection named "Running a command at build time". And it barely tells you how to do this at all (this being a topic I've actually struggled with in the past). In particular it fails to mention that a target is is not the same thing as a file in cmake, and that this has implications that are likely to be surprising to make experts.