Do-it-yourself (DIY) builds describe how to assemble a vehicle from parts that are sourced separately.
At the easier end of the scale of DIY builds are those that take an existing RC vehicle and graft on a flight controller. At the harder end of the scale are "unique" vehicles where some parts may be fabricated using a 3D printer. Somewhere in the middle are builds that use separate but readily available components: frame, motors, ESC, propellers, flight controller, and so on.
DIY Builds: