Pipes are normally supplied in either single random length or double random length which are circa 6m & 12m respectively or can be specified cuttings from these lengths at extra cost with smaller bores in the SRL for handling purposes with lengths I expect being standardised for handling and transporting. The joint will be to type specified in your piping service class which may be threaded, socketweld or buttweld with typically NPS 2 and over buttweld. Typically threaded piping is only used in utility systems and is joined by threaded couplings in straight lengths and threaded elbows, tees etc in change of direction with socketwelds being similar except for type of joint but are also used in hydrocarbon services depending on client. Again buttwelds have similar fittings for change in direction with obvious different joint type but straight lengths of pipes directly welded with no couplings.