There are two ways in. With primitives you stack boxes, cylinders and spheres until the shape is right — fine for a mount, a spacer or a bracket. For a profile that standard shapes won't give you, draw a sketch first and pull it up (extrude) or spin it around an axis (revolve); a knob, a wheel or a bottle appears in a single move. Model at full size in millimetres from the start: a 3D model carries no unit of its own, and your slicer will later assume one unit means one millimetre. Everything runs in your own browser, so your design stays on your machine.
You don't make a hole with an eraser; you subtract a cylinder from the model — a boolean. Need a row of fixing holes? Subtract one and repeat it with a pattern: quicker, and the spacing stays exact. Then measure, because the number in your sketch is not the size that comes off the printer. A 5 mm hole for an M5 bolt will bind; count on two to four tenths of a millimetre of clearance. Unsure about a thin wall? Check the stress with finite elements before you burn hours of print time.
You export STL for the printer: that file contains only triangles, exactly what a slicer needs. 3MF does the same, but also carries the colour of each body, and most modern slicers read it. Use OBJ and glTF when the model goes to a rendering or 3D program. If a colleague needs to keep editing the real shape in their own CAD package, they need a STEP file: you can open one here, but not create one, because everything the modeller exports is a mesh of triangles. Mind how fine those triangles are: the rounder the shapes, the more triangles and the larger the file. If it no longer fits in an email attachment, send it as a download link.