I worked at a great startup before starting my studio.
Most of the people who worked there ended up moving to much bigger companies. Most of them were happy to introduce me since they liked my work. That's how I got to work with large startups.
I have tried Upwork and Fiverr, but much later into my career. It was so late that I couldn't justify the time and effort that it would require to land a client there (you need testimonials/reviews to build a strong profile, might have to take on lesser paying jobs just to get those ratings).
If I had to start all over again, I would definitely give fiverr/upwork a proper shot. But it doesn't make sense today.
(Also, I read somewhere that platfoms like fiverr/upwork is a race to the bottom where clients filter based on lowest price. If that really is the truth, I don't want to compete there either)