Background
Most manufacturing organizations will require a multi-level Bill of Material. Meaning that it needs to manufacture certain Sub-Assemblies before it can manufacture the Finished Goods that can be shipped. ERPNext enables you to create a Multi-Level BOM where you can define a nested structure of BOM.

This video considers an example of a Weather Sensor and explains how to create a Multi-Level BOM for Weather Sensor with Operations. BOM can also provide an indicative Costing of the Product including Operations Cost and Raw Material Cost.

How to Create a BOM
Create BOM of an Item using Bottom-Up Approach, meaning create the BOM of the most basic Sub-Assemblies required to manufacture Finished Goods first and then move upwards. The reason being, reference of the most basic Sub-Assembly will be required in the BOM where it is used as an Input. Using this approach, you can create Complex BOMs on ERPNext with ease.