Add a cube, a sphere, or another shape from the Tool panel, then click it to select. Press G to move, R to rotate, and S to scale, and press X, Y, or Z to lock the transform to that axis. Hold Ctrl while dragging to snap.
The Properties panel adjusts position, rotation, scale, and dimensions, along with the material color, metallic, roughness, and texture. The Scene panel switches the background color, grid, shadows, lights, and camera view. Save and load your finished scene as JSON from the Tool panel.