You're right, every site is going to tell you something different.

Since there isn't any "right" place, you just need to pick one place and use it consistently.
My gym teaches the personal trainers to measure the smallest part of the waist because that's easy to locate on most women. But for those who have a lot of belly fat and may not have a smallest part, it's easier to pick a landmark like the belly button. You just want to pick a place that it's easy to locate
on you every time you measure.