That is a really good point, and there isn’t a one answer first all. If you’re looking for a mobile app, then you could do Android studio or Swift for IOS. If you only want to develop one app for both, then maybe you should just do an electron app that is essentially just a website.

These tools will either have a visual interface editor, markdown language (like html), or a way to define visual components in code. After programming an app you can build, which will give you the appropriate executables for the different platforms.

If you’re looking to start simple, then use something like pygame to learn the basics of using a visual interface.