Add it to HEXOstudios, free on the Mac App Store. HEXOstpEngine turns the file into a thumbnail on its card and a 3D preview when you select it, in the file’s own colours; with Pro, Full Preview opens it full-screen. You don’t need CAD software just to look at a STEP file.
There is none: .step and .stp are two file extensions for the same STEP format, and HEXOstudios opens both.
No. HEXOstpEngine processes every file on your Mac, inside HEXOstudios. Your STEP files never leave your machine.
No. HEXOstudios is a library and viewer: it shows STEP files as thumbnails, previews and 3D, with their STEP Passport, but doesn’t edit them. HEXOforge converts STL, OBJ, PLY, 3MF and AMF files; STEP files aren’t converted.