Quote
why a 1/2" thread has a diameter of about 3/4"
The nominal diameter is the internal one and the external dimension includes the thickness of the pipe walls. I came across a curious difference in the US where brass compression fittings are quoted using the external diameter of pipe! I couldn’t find a ¾” union (for ¾” copper pipe) I needed until I was directed to the 7/8” ones.

The metric world adopted the British Standard Pipe (BSP) thread as the ‘metric’ standard. The diameters are sometimes quoted in mm but the thread pitches are in rational units of threads per inch (tpi) – that should make Paul smile [Linked Image].

The US of course uses National Pipe Thread (NPT) which has different thread pitches than BSP thus they are once again different from the metric world.

Then there are the lamp thread pitches just to keep this subject on topic. The same threaded tube for table lamps etc is used in both South Africa and the US. It is called 1/8” ISP in the US but has a diameter of ~3/8” (~10mm) and a thread pitch of 27 tpi. In this case the internal diameter is well in excess of 1/8”.