These are just a few links to my favorite YC videos:
My favorite video so far: Caring is everything
Second favorite: It's nice that some of the videos mention ethics and happiness, like this one. There are so many entrepreneurs that are morally bankrupt, and I like these guys way more for recognizing that's not a great way to be, or even a helpful ethics under which to operate, in most cases.
Closely related to those two: On building conviction, avoiding a random walk and pivotitis, and developing a real MVP - it's going to take way longer than you think to find product market fit and build a customer base that loves your product - start by being your own customer that loves your own product - build conviction in your own mind that this thing is worth working on - it can be hard to build, but if you love it, and you love your customers, then you are on the right track.
If you have anything like a technical product - DO NOT OUTSOURCE YOUR TECH - this comes up again and again in Y combinator video series and I suspect that everyone who has been in the startup world for more than a decade cringes when they hear of entrepreneurs who outsource technical products.
My favorite series so far: Y combinator partner lounge videos