OK, here's the thing. If you come from DC electronics, you're really accustomed to being able to route the + wire any way you want.... and the - wire any way you want... as long as it gets back to the battery or whatever.
That's not how it is with AC power. AC throws a considerable AC magnetic field because the current is considerable. Exploiting that is how transformers work. Those magnetic fields can cause lots of trouble if they are not managed. They can cause vibration in wires, and since copper and aluminum do not have a fatigue limit, that will crack them, causing hotspots or arcing. They can cause eddy current heating in metallic things nearby (including non-ferrous; inducing onto non-ferrous metal is how induction motors work).
So, in AC power installations it is required that current return the same way it came. This causes the equal and opposite magnetic fields to cancel each other out. All related conductors must be in the same cable or conduit. Documented in NEC 300.3.
A cable is several wires wrapped in a sheath.
So for instance if you have a bathroom fan+light, powered from the dual switch, you can use one /3 cable with red+black for the two switched hot wires, and the white for neutral. Whichever current goes up to the light/fan returns on the neutral, and so current is equal & opposite on the cable in all conditions.
Neutral MUST travel with the hot wires it is associated with. There is no option for running a network of neutral wires separately.
As a practical matter, this means cables in homes is in a tree topology. They can fork at any point, but they can never rejoin to form a complete "loop".
When you have power sources entering a box from different directions, e.g. violating the "no loops" principle, then you must keep each group of neutrals completely separate from each other. It's not required to use a physical box divider, but for novices it's not a bad idea.
