There is only one reason to join a startup as an employee: you really really really believe in the mission and you reallyx3+ believe that the team can execute. Anything else and you aren't doing the team or yourself any favours.
So, what are you really applying for?
So you can have a "worked at a YC startup" on your resume:
Just go work for google or facebook or amazon: you'll actually learn a tonne more about engineering/product dev at any of those places and they are a much better brandname for the rest of your career. Think you wont get into those companies? Spend a month preparing for the rigamarole that is a technical interview and you'll be fine.
So you can get investor contact:
Just apply to YC/techstars/500startups. Or email investors directly. Any of those options work better. You aren't getting any meaningful investor contact by being an employee.
So you can learn how a startup works to start your own:
Anyone who tells you this is deluded or conning you. There ain't nothing to it but to do it. The only way to learn how to do your startup is to do it. Nothing else comes close. Anything else is an excuse.