First of all, it's much better to follow the official guides, videos can become outdated any day https://grapheneos.org/install/web
It was very easy to follow and it was done very quickly, everything has been working fine since.
For the most part warnings are for you to be aware of what can happen, but often the steps are really simple and the software has been tested many times making it very unlikely to fail.
I bought a pixel 7 pro from my carrier and it wasn't locked, I think recent pixels aren't locked anymore. If you buy an old one then you might need to verify it first, I'm not sure how.