As has more or less been said.
Course Forge basically lays a blanket over your terrain where you draw out the splines for tees, fairways, rough, bunkers and greens, then position your tee boxes, pins and hole aim points and the remainder is all Unity, which you are already well on the way to familiarizing yourself with.
You can more or less layout the course in Unity alone, but Course Forge is needed to make the courses playable by adding tees and pins. Without this key element of Course Forge, Unity only designed courses would never be playable in Perfect Golf.