You can have a big idea that makes no money (like Quibi, a streaming service that raised 1.75 billion and shut down after 6 months). You can also start as a door to door salesman selling baking powder and accidentally create Wrigley's gum. You can also assemble custom computers as a side hustle and then scale the business to become Dell.
You have no clue what's going to become a big business. You have no clue what idea will require a lot of work, and what idea will just go viral. Candy Crush makes more money than No Man's Sky.
The thought that you can predict what idea will make it big is laughable. VC's invest in hundreds of companies because while they're pretty good at picking companies, they know it's largely a crapshoot.