InDesign is print-oriented. You cannot really work in pixels, it has no idea what size pixels there will be. (Unlike Photoshop, which is a raster file and lives in the pixel world).
So, when you choose a size in pixels, InDesign simply converts to a real size.
When you export as JPEG, clearly the design must be rasterised. The resolution has no connection to the resolution of any graphics placed, but is chosen on export. The rules are very simple e.g. if you have a design 10 x 10 inches and export at 300 dpi, you get 3000 x 3000 pixels. If you export the same design at 72 dpi you get 720 x 720 pixels.