When you created the second project, you seem to have accidentally created it inside of a folder in the first one.
This can happen if the solution (CoreApps.sln) file is in the same directory as the first project file (FirstCoreApps.csproj). The dialogs then probably suggested a directory that you should not have used.
Since the new project model automatically includes everything in the folder of the csproj file, the files of the second project automatically become part of the first project.
When creating new projects, be sure to check the "Create directory for solution". This creates a folder structure where the .sln file is created in its own folder and ever project in its own sub-folder.
