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James Westman recently shared how he added mobile app development capabilities to GNOME Builder. Take a look at his blog post for all the details: https://www.jwestman.net/2021/08/06/finishing-gnome-builder-mobile-support.html
Or jump right into setting it up yourself with his tutorial: https://www.jwestman.net/2021/08/06/tutorial-mobile-dev-with-builder.html