things just grow on their own sometimes, especially if you're successful enough.

it's perfectly reasonable for a multi-billionaire to want to write a blog to publish his thoughts on various things. the existence of "the gates notes" is pretty normal. and when you're rich, you hire somebody to run the website for you. web design and marketing are pretty interlinked, so the guys running the blog do some promo, and generally run it the same way they would run any other content site. which includes making some video content targeting the most widely-viewed types of posts.