How do I write my business plan?

This isn’t a general question on how to write a business plan, it’s more-of how do I go about it in the following scenario.

I own a web design company that’s transitioning into a software development company. I already have the first software laid out. A business plan was never written for the web design company, so….

Assuming I’m looking for investors, team members, etc. Do I write my plan for the parent company, or do I write my plan for the software itself? I would be pitching the software product and not the company itself.

I’m confused on how to organize a software company in that I understand software is a product but they can also be their own entities, but if all software is under one company (like Adobe), then how is everything organized and how are business plans written and pitched to investors… for the company or for the product?

I hope this question is clear enough :)