Isometric is a projected 3D view defined by the camera position (viewpoint), without perspective. Back in the days of first-principle paper drawings, drafters used rulings from orthographic projections to derive isometrics, but TC and other CAD programs don't derive 3D views directly from 2D projections. They use camera position relative to 3D models, which gives an infinite variety of views and their projections. Isometric and orthographic 2D are just some of the conventional ones.
If you've made a 3D box, the isometric views (there are four standard ones, NW & NE, SW & SE, all relative to the global workplane, but you can also define your own coordinate system and planes, and use them) look at the box from combinations of 30/60 degree elevations and relative azimuths, and viewports based on them can be inserted in paper space, which is 2D only. In viewports, you see 2D projection of 3D viewpoints, with or without perspective, not 3D projections of 2D views. It's a bit like drawing on your window pane with a marker pen to trace the view outside.
