When I first applied to YC, I used iPG (https://jamescun.github.io/iPG/) to practice and get comfortable with the YC-style interview. Spoiler: I ended up getting in.
This took me a few hours to go from scratch to production. Would highly suggest playing around with some of the latest products (this uses Vapi.ai/elevenlabs.io)
Show HN: Practice for the YC interview
mock-yc.vercel.app