Assemblies are not plant-specific. They are considered design/phantom/grouping type of materials. They aren't physical materials but rather a grouping of physical materials.
The idea is that you create an IBAU for each of these "groupings" of parts then you create a material BOM (or rather a maintenance assembly BOM) in CS01 like you would normally do for a typical material BOM except that the parent or source material is your new IBAU.
Next you would create an equipment BOM where you would add your IBAU as a component of that equipment BOM. Since maintenance assemblies (aka IBAUs) are not plant-specific you must add the IBAU to the equipment BOM as a PM Structure Element (BOM Item Category of "I").
If you want to go the route of using construction type then the idea here is that you would create a material BOM similar to the equipment BOM. The difference is that instead of an equipment BOM pointing to material BOMs you would have a material BOM pointing to other material BOMs where the master material of this whole BOM strcuture is actually a material representation of the equipment.
For example, let's say you have 5 pumps with identical BOM structures. You would create a material called PUMP then proceed to create the BOM structure for a typical pump. After this you would create your 5 pumps as equipment and assign them a construction type of the PUMP material. In this way your equipment will now inherit the PUMP BOM structure.
The benefit/drawback to using construction type is that your BOM structure remains the same for all equipments using that PUMP construction type.