You'll need two abilities, being able to write the app and to sell your app services to businesses.
They just need one ability, the ability to sell the app services to businesses. Everything else will work itself out.
Some people build plugins for ecosystems like Wordpress and that's pretty profitable.
I have a Slack marketplace app that is ramen profitable.
Write code to solve a problem and charge for it is probably the best way to monetize your programming hobby.
Excluding the rare possibility of making a top tier app, I feel like it's a more lucrative option to do yardwork. I'm a beekeeper and would guess I can average $500/yr with that.
a hobby seizes to be such once it's a business. i suppose for each the distinction between the two can be wide or narrow but it exists.
a business inherently involve transactions and this implies vastly more amount of responsibility.
I just took up drawing as a hobby without any expectations that it would bring me money, and its been much more enjoyable than other hobbies I've had where at the back of my mind I was thinking 'how could I make money off of this'.
https://www.aicrowd.com/ https://www.kaggle.com/competitions
Sites like https://www.hackerone.com/ is basically companies daring you to break their stuff, for money.