Earlier quoted context omitted.
You can easily start a company that requires significant R&D: start with consulting; then one day sell the product you want to make with the right license and timeline, then make it and sell it to everyone. Look at Space X, they were making money wayyy before they were igniting rockets.
I don't think you could pick a worse example. Space X was founded by a billionaire, who injected 100M USD from his own pocket, and then took 20M from his buddies. That's as far as it can get from what people imagine when they read "bootstrapping".
More importantly, the goal of SpaceX is fundamentally the colonization of Mars, and the idealized method to do that is a fully reusable interplanetary spacecraft. If SpaceX had tried to build that and only that from their start they would have rapidly ran out of money, even $100 million, and had little to show for it. Instead they bootstrapped their way toward acquiring the capability to design, build, and operate such things while also building a corporation capable of funding such operations. Initially they built a small orbital launch vehicle, just barely at the limit of market feasibility, using very conventional technologies and very conventional designs, applying innovation peace-meal where it made sense. Even then it took them several tries to get a launch vehicle that worked, and it wasn't even very competitive in the launch market. But it proved that SpaceX was capable of building launch vehicles, which enabled them to get grants and contracts to build a launch vehicle that was.
It wasn't a demo or a prototype, the Falcon 1 was a fully functional "MVP". And it wasn't investment funding that got them to the next stage, it was effectively pre-orders for the next generation of their product. Then you look at what they've done since, they've built iteratively, advancing toward their goal, but even now they still haven't gotten everything into place that they need. They're still working on partial reusability, still working on manned spacecraft, still working on clustered core staging, and so on. But even so they have viable products that are bringing in a lot of revenue, with new products coming online soon that will bring in even more revenue. They are still several iterations and many years away from their goal but by the time they get there they'll have the expertise, experience, and revenue to actually make it happen.
As with most endeavors, it's almost always easier and better to get to a big goal iteratively. That's how you build skill, it enables you to sell your earlier iterations to maintain revenue to keep your company alive, and each iteration provides lessons which inform the next and can reshape the grand idea you once had.