Something I've learned from working with a lot of YC companies, is that while they certainly all want people who are "smart and motivated", they measure and define it quite differently.
It sounds like your concern is whether companies are only looking for c.s. grads from brand name schools and companies, and I can specifically vouch for that not being the case. Depending on the company, they're likely to value open source contributions, independent projects you've built that are interesting, startups you've started or worked at, impressive technical work you've done at any company, and how candidates do on their own technical interviews (which vary greatly in substance), among many other factors.
It sounds like your concern is whether companies are only looking for c.s. grads from brand name schools and companies, and I can specifically vouch for that not being the case. Depending on the company, they're likely to value open source contributions, independent projects you've built that are interesting, startups you've started or worked at, impressive technical work you've done at any company, and how candidates do on their own technical interviews (which vary greatly in substance), among many other factors.