For questions not addressed here, inquiries about video sponsorships, or anything else, you can find my contact info on the about page from Youtube (it's just my first name @ this url).
Question: What do you use to animate your videos?
Answer: I create the animations programmatically using a python library named "manim" that I've been building up. If you're curious, you can find it at https://github.com/3b1b/manim, but you should know that I developed it mainly with my own personal use case in mind. It's not that I want to discourage others from doing similar things, quite the contrary, but often my workflow and development with manim can make it more difficult for an outsider to learn than other better-documented animation tools.
There are aspects of producing videos in a software-driven manner like this that I find quite pleasing, but which are pleasing precisely because it's my own tool. It enforces a uniqueness of style, for example, which is by its very nature a benefit that can't be shared. There's also a certain freedom in being able to tear up the guts of the tool every now and then when I feel a change is in order, since backwards compatibility needs are very limited when you only care about videos moving forward. Not exactly the best practice from a collaborative standpoint.
All that said, if you do want to try it out, never hesitate to ask question, or to let me know about things that can be improved.
Question: What does the name "3blue1brown" mean?
Answer: When I started the channel, I knew that I wanted the logo to be a loose depiction of my right eye color: sectoral heterochromia, 3/4 blue 1/4 part brown. It was a way of putting a genetic signature on my work, and the channel is all about seeing math in certain ways. The name, of course, is just derived from the logo.
Question: What's the music playing in your videos?
Answer: For several I've just written my own simple background music. The piano song throughout the Essence of linear algebra, for example, is not actually a complete song, just enough musical doodling to fill in the intro/outro. Several recent videos have used a song kindly written by Vince Rubinetti.
Question: Who are you? What's your background?
Answer: My name is Grant Sanderson. I studied math at Stanford, with a healthy bit of seduction from CS along the way. For a while, my job experience was pointing me in the direction of software engineering/data science, but ultimately the primary passion for math won out at the expense of the mistress.
I've loved math for as long as I can remember, and what excites me most is finding that little nugget of explanation that really clarifies why something is true, not in the sense of a proof, but in the sense that you come away feeling that you could have discovered the fact yourself. The best way to force yourself into such an understanding, I think, is to try explaining ideas to others, which is why I've always leaned towards the teaching/outreach side of math.
I was fortunate enough to be able to start forging a less traditional path into math outreach thanks to Khan Academy's talent search, which led me to make content for them in 2015/2016 as their multivariable calculus fellow. I still contribute to Khan Academy every now and then, as I live near enough and we remain friendly, but my full time these days is devoted to 3blue1brown.