A friend of mine has an eCommerce company of ~30 people. They're now collaborating with an Odoo Partner to implement Odoo as their ERP. He wants me to become their Odoo Developer and Maintainer in the future. He wants me to not just know how to code, but also understand the parts the business is running. From where should I start learning?
Your friend could walk you through their business processes and how they plan to use the third-party software. It would be a good chance to document all that knowledge.
You could also take a look on the documentation of Odoo.
I haven't had first hand experience with odoo. But their GitHub repository would also be a good place to start.
They have a tutorial : https://github.com/odoo/tutorials