If you microwave bananas for a few minutes in a bowl covered with plastic wrap and then dump everything into fine mesh strainer to sit for about 15 minutes, you will get a bunch of juice out.
Freezing the bananas before microwaving them works even better.
This technique was used in a (really good) banana bread recipe from Cooks Illustrated several years ago.
Once the juice is extracted, it is reduced, allowing more real bananas in the bread without making it too wet.
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I've made banana juice a number of times.
And I'm referring to a relatively clear liquid extracted from the bananas.
It's a pretty straightforward process that produces a high yield.
The water in bananas is chemically bound to the starch in bananas and this liquid can be released by breaking down the starch using an enzyme called amylase that is present in bananas.
Here's how I do it: (Yield 1 liter of juice) Take 2 kilograms of bananas, peel and puree them.
Pour the puree into a vacuum bag and seal.
Alternatively, you can put them in a VERY securely tied plastic bag with the air pressed out or you can put the puree into a canning jar.
Next place this in a pot of water at around 150 Fahrenheit/65.5 Celcius and let sit for 5 hours.
This allows the naturally occurring amylase in the banana to fully breakdown the starch in the banana and release the liquid.
Strain the liquid through a nut milk bag (my preferred method), or you can use several layers of cheesecloth, but a nice nut milk bag makes this so much easier.
Squeeze (and massage to get the highest yield)
the juice through the bag.
You should get about a liter (generally a bit more than that) of fresh, delicious banana juice!
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The people of North West Tanzania and Southern Uganda have been making alcohol from banana juice for years.
They blend ripe bananas with grass and mix with there feet or hands until the enzymes break down to get clear juice.
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hmm...
I imagine that a method similar to getting liquid to make jellies could work for this.
Freezing would break the cell walls and allow more liquid out, then you could blend it to a lovely mush and then put it into a jelly-bag or cheese-cloth and suspend it over a bowl in the fridge for a day or two and you should get some liquid out :)
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You can make banana juice by using enzymatic breakdown of the starches.
It is not a difficult process, and can be accomplished in the home kitchen, but would require some ingredients and techniques that you may not have or be familiar with.
The details can be found here: https://www.chefsteps.com/activities/banana-juice--11
While this produces excellent results, you could speed the process greatly and achieve a higher yield if you have access to a food safe centrifuge.
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I have looked up how to make Banana juice because I wondered if it was possible. It turns out that it is.  I read that it is best to use a blender for this.  Wouldn't I just end up with a banana puree if I use a blender to try to juice it?
How to make Banana juice
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Bananas can be frozen to break the cell walls or microwaved for a few minutes before being strained through a fine mesh strainer or cheesecloth, allowing the juice to drain out. Modern methods of extracting banana juice involve a number of processes and machine. The traditional method is to use grass and your feet.
Microwaving and freezing bananas seem to be the modern options for extracting banana juice, followed by processing and straining. The traditional method is to mix bananas with grass and mash using your feet until the enzymes brak down.