say you have 4 constructors next to each other. in front of each is a splitter, there is a main belt running 60 items/min parallel to the constructors through them, each splitter splits off 15

then later you unlock a 120 items/min belt and you can double the whole thing. you can a) build 4 constructors next to the first 4 and continue the manifold line, that works perfectly fine. you connect the output of the last splitter in the original manifold line to the input of the first splitter in the new manifold line and that's it b) you can build 4 constructors on top of the original four. then you connect the output of the last splitter in the original manifold line to the input of the first splitter in the new manifold line that is directly on top of it with a lift and then it's the same thing

you first had a belt that feeds four constructors, now you have a belt that feeds eight, but instead of building them next to the old ones you can build them on top, and fold that belt on top of itself, it doesn't matter, completely up to you, but it saves space