These two files control repo-wide build settings.

Msbuild projects work by including a "default" props file, then allowing projects to override those defaults, then including a "props" file that can apply high-level properties into lower level configuration. After that, projects can once again override lower level values, then define which files to compile, then include a "targets" file to perform execution.

In other words, the way to think of these two files is Winfile.default.props executes before Winfile.props, and the two files are split to allow individual projects to manipulate state in between.

Generally, anything affecting supported architectures, compilers, and basic compilation options should be here. Anything specific to an executable should be in the project files.