No, ads will not show in videos if a channel hasn't been accepted in the YPP unless a third party claims the video (copyright) The reason why you want to enable/tick that box is because when your channel gets approved, after that moment ads will start showing up and you will see a bunch of green monetisation icons that look really cool when you see them the first time. If you decide not to enable ads in your videos, once you get approved you will need to do that manually which depending on how many videos you have it might be a pain to do... so. I recommend always tick that box and every single ad that appears there and let the magic happens once you get approved!
You haven't uploaded in like 3 months, you are so close! Don't give up... you got this!
