Most likely the trackpad cable. On the 2015 models, if the trackpad cable is not plugged into the logic board, the keyboard and trackpad will not work. I have come across a few where the keyboard and trackpad will intermittently not work, reboot usually temporarily fixes it, but in all cases the trackpad cable has been the issue. Sometimes the cable looks fine, sometimes it has slight liquid damage.
It is very easy to replace (remove bottom cover, disconnect battery, then disconnect the cable) and it is very cheap. Try this first, and it should sort out your problem.