@NickyA708 The official CraftBukkit downloads from Bukkit are no longer availible because of a DMCA takedown in 2014. However, spigot have made BuildTools, which will compile craftbukkit on your computer for you. The reason that it compiles it on your computer is that the code which got a DMCA takedown was only disallowed to be distributed in compiled form, so BuildTools is sort of a workaround. (Don't quote me on the fact that anything of this is legal, I am not a lawyer)
@AlvinB This isnt legal, Buildtools is not a workaround and recieved a DMCA itself for distributing code that they have no permission for. Theres a reason its using a safe-haven website.