Funding for 3blue1brown comes mostly from community support through Patreon. The patreon community has also been a great source of early feedback on videos before their public release, especially while working on series. There are also several perks and benefits to members, outlined on the page.
This direct-contribution model has, I believe, changed the content itself for the better. It means incentives are aligned towards creating what reaches people most deeply, not what reaches the most eyeballs, so I am extremely grateful that this model is even possible, and for those of you who make it so.