They majorly select a kind of founders as a first filter (good engineers from good companies, ivy league, ex founders etc.), and then they look into the ideas as a second filter, and the third and the least important is the process in which they just spend 15 minutes interviewing and maybe another 15 minutes looking into the application.
The zeroth filter is 'people who apply to YC' though, which YC can only really control by managing their brand and marketing and by approaching potential founders directly and suggesting they apply (I assume that happens; I don't know though). That limits who they can invest in far more than any of their own criteria.
There's also another way in that circumvents all the other filters - being a founder at an existing startup that has really good traction already. You can have a resume of no-name companies, no degree, and never have founded a company before, but if your business is growing, making money, and looks wildly scalable with YC's support then you can get in that way.
