So the main GPU's that are being made are by Nvidia and AMD. They design the processor of the GPU and they draw a layout for the PCB that all the components sit on. They then send this information or the parts to individual board partners.
The board partners, or as you say aib, then decide what to do.
1. Keep everything the same (reference version)
2. put a new cooler on it to improve themals and keep the pcb the same.
3. create a new layout on the pcb as well as a new cooler over reference design.
Obviously option 3 can have the most benefits, improving on many of the things Nvidia or AMD might have done a certain way. These changes can include different outputs on the back of the card, more power inputs (6-pin or 8-pin) that will allow more power draw, and some others.
Hope this helped you understand a bit.