Does youtube use phone data when you're watching videos on wifi?

I just did a test on my phone to see whether that’s true (in my case), but it appears to be not
I have wifi and mobile data both open and played a YouTube video.
Before and after I checked how much mobile data was used; no difference.
If you want to be 100 percent sure you can go to your settings and turn mobile data usage, per app, off.
Depending on OS and brand, you most likely will find said settings under Wireless and Networks, then Mobile/Cellular Data, then Apps in Network (or something similar).
In this tab you can choose WiFi and/or Mobile Data to be On or Off for all (pre-)installed apps.
Hope that helps.

It can use your “data plan”, if you have a weak WiFi signal.

Both @rebbel and @Tropical_Willie are correct, AFAIK. I asked this question of one of the “geniuses” at the Apple store, and they said that if you have a WiFi connection, your phone will always preferentially use that for all apps. But as @Tropical_Willie said, if your WiFi signal is weak, it may switch to phone data. However, there is a setting where you can override this and disallow switching over to your data plan.