Since we don't have this documented anywhere, I'll explain real quick.
Weld, Snap, WeldConstraint, Motor, and Motor6D joints all combine multiple parts into the same Assembly. An assembly is a rigid body if none of it's parts are anchored. No physical forces can ever separate the parts of an Assembly or move them relative to each other unless the joints are removed or updated. They're a single body.
Every Assembly has a root part, see BasePart:GetRootPart. When a weld is C0/C1 is modified the root part will stay where it was.
Welds do not have any directionality. Part0 or Part1, doesn't matter. You can imagine rigid joints forming a tree branching down from the root part. All the parts down the tree from root will move, and their welded "children" in this tree will move with them.
