How to compile a C++ project for different platforms?

Question

Qt

Let's say I want to create my own simple framework like Qt, so that I can compile C ++ projects for androin, ios etc on it. How it works? What do you need to know for this? How difficult / time consuming is it? Answers to any of these questions will do.
And is it possible to compile "hello word" on android without making a titanic effort?

Answer 1

is it possible to compile "hello word" on android without making a Herculean effort?

The easiest way to do this is to build the application with a linux compiler on the target CPU statically.

Answer 2

Each target OS needs its own compiler, which in turn is compiled for your working OS to create binaries for the target OS.
You can read about crosstool-ng or get a compiled gcc compiler for your needs.