To create a 2-level product hierarchy in Odoo, you have a few options:

1. Parent-Child Product Categories: Extend the existing product category model to support multiple nested levels (this requires custom development).

2. Custom Fields or Tags: Use custom fields or tags to categorize products across different levels of hierarchy, giving more flexibility.

3. Multi-Category Approach: Assign a product to multiple categories, allowing it to appear in different hierarchies, which doesn't create a strict hierarchy.

4. Custom Module: Develop a custom module to support recursive parent-child relationships for unlimited hierarchical levels.

5. Attributes and Variants: Use product attributes like “Brand” or “Color” to create a more structured classification, though it's not a strict hierarchy.

There is no hierarchy.

Any product can be a top level or bottom level. Any product can be sold. Any product can be manufactured or purchased. Any product can be part of any BOM (exept it's own BOM).

Any product can be any of the above at the same time, so there are no limitations.

If on the other hand, you want to establish some tracking on your products based on information you know about them, you can use the category field, the tag field or create a custom field.