Probably the best would be to learn a bit about how to use the Segment Editor by following these turorials: https://www.slicer.org/wiki/Documentation/Nightly/Training#Slicer4_Image_Segmentation
They explain step by step how to use scissors and other effects.
Recent nightly versions contain direct export to file feature: Save segmentation directly to STL or OBJ files
Operating system does not matter: all features are available on Windows, Linux, and macOS.