What kind of app are you looking to build?

If it is a general purpose application, I'd look at learning rails (this book was great for me: https://www.railstutorial.org/book ) and host on heroku.

If it is an API driven application, I'd look at using a tool like Zapier: https://zapier.com/ or Transposit: https://www.transposit.com/ (full disclosure, I work at Transposit).

There may be some low code/no code solutions for mobile apps, but I'm not in that space so not familiar with them.

Frankly, depending on your app, you could do a lot with google forms/typeform and some manual behind the scenes work: http://paulgraham.com/schlep.html

Again, it all depends on what kind of application you are trying to build.