Nearly all YC companies that are smaller post their job postings on HN. Check here:
https://news.ycombinator.com/jobs (it updates frequently, check it daily).
That said, if your goal is to become a technical cofounder, you are likely going to have a hard time. Very few YC companies are looking for a cofounder, and if they are they would likely look in their personal network first. You mention being an early architect, which is possible, but maybe not before major funding (depending on what you consider major).
Nearly all YC companies avoid hiring during YC, and instead hire after demo day. This is mostly because hiring is insanely time consuming, and during YC they focus almost exclusively on growth and getting the company in the best shape possible for demo day. Many companies also don't have the finances to hire someone until after raising, which happens after demo day.
Regardless, the HN job listings is your best bet.
