From the embedded video [paraphrasing]: "We've been exploring this idea for the past 3 decades." I've always wondered how people are able to sustain themselves

From the embedded video [paraphrasing]: "We've been exploring this idea for the past 3 decades." I've always wondered how people are able to sustain themselves financially for multiple years working in a start-up without generating any income. Is the payoff so big that investors just continue to inject funds? Is it necessary to get grants? What kind of salary can you sustain just working on ideas for decades without any actual product sales?