How to wheelie without hands? I have seen videos of people riding wheelies and letting go of the handlebars.
I have tried doing the same but I have no idea how to do it.

I can ride wheelies easily on my mountain bike.
I am well in balance and not accelerating but everytime I try to let go of the handlebars, the front wheel just goes back down. To keep the wheelie up, I am constantly pulling the handlebar towards me to keep the balance.

So how is possible to ride a wheelie without pulling the handlebars?
You need something else to prop your weight against. Artistic cycling practitioners usually use saddles with high rear edge that can be leaned at. And of course you can use other parts of your body, such as shown here (check the rest of the video for the saddle).