You simply copy and paste it into the new project.
If you are using Stacks 4, they’re also Externals. These are similar to partials that work across projects. However, if you make changes in one project, those changes will be made across all projects.
But when I try to select it the ‘copy’ option in the menu it is greyed out, on a similar theme, Cmd C and Cmd V often do not work in RW, I often have to resort to the Edit Menu.