Recording a video from your iPhone?

I have been looking at some videos on youtube like vlogs recorded from their iphones and other when they record the whole screen isn't filled only that small part. So how do you record on your iphone so that when you post it onto youtube it'll cover the whole screen not a part of it?

I'm pretty sure you just record with your iPhone horizontally.

Rotate your phone 90 degrees until it's horizontal. Basically, you record while your phone is sideways.