Add a new subproject to a kotlin multi-module project

I followed the docs for building a creating and building a new kotlin-based multi-project build.
Now I’m trying to append a new subproject (The core library of the project that i’ll reference via other clients interfaces e.g., command line).
I thought about simply creating a subproject directory, including its in the settings.gradle.kts include block, and from within it, I’d create a build.gradle.kts file with the respective directory structure within the subproject as a whole, but I thought there might be a better way.
Is there any built-in way of achieving this?

You mean an “add another subproject” command that does it for you?
Not that I’m aware of, at least not built-in.

so what’s the optimal way of doing it manually? (In the context of kotlin-based projects)

Well, probably what you said yourself.
Add it to the settings script, create a build script and you are done.
Optimally, you would have convention plugins so that you then just apply that one plugin in your new project and maybe add some dependencies.