Doc most likely used any leftover part & the main source is the Hoverboard that thing is insanely advance my guess it gave Doc all he needed for one trip so Doc went to the future to get everything for a proper time machine then he got his family & the rest is history & God bless 🙏🏾🤟🏾❤😄

Doc used the the leftover parts & opened up the Hoverboard that gave Doc all he needed to make one trip so Doc most likely went to the future to get all that he needed to build a proper machine then got his family & the rest is history God bless 🙏🏾🤟🏾❤😄