Business plans used to be really popular, but at least for Startups (and VC) it tends to be pitch decks (the Y Combinator one, you can google is particularly good)

whether you do a pitch deck or a business plan in a more conventional sense you still want to cover areas such as. - competitors (and features and why you are better than them); total market size? beachhead of the market, the customer personal, you want to articulate the problem, how your solution solves the problem, how your business will make money.

personally, what i find more important than a pitchdeck or a business plan is the financial model - i was speaking with a fellow VC at a conference and we agreed, if the financial model is sufficiently good, we can understand most of what the business is trying to do, how it will do it, how it will make money and how it will grow, so then we only need addressable market size data, and verification that the founders assumptions are realistic and achievable - TL;DR - do the financial model first and use it to plan.