Another business major here. I don't have an online course to recommend, but the BUSN book basically taught me all I needed to know to get started in business administration. https://www.amazon.com/BUSN-Marcella-Kelly-ebook-dp-B079QD31...

After a decade working post graduation and nowadays being comfortable negotiating and managing millions-dollar projects, I still find the book covering all the fundamentals you need to know about modern business.