Good approach to develop C++ program with intention to port it to mobile on Android and iOS and Web

Recently I have had strong interest in C++ because I have some programs in mind that may need intense calculation or evaluation. I intend to move the program to mobile once it's developed. I had abandoned the idea to do it using Python or React and decided to do it using C++ for its high performance. It's a big challenge but exciting to me. I had learned C++ many years ago but almost forgot all of it for not using it. I have always been with Java so that would help a bit.

I browsed the web and found the CMake is the way to take C++ into both Android and iOS. But it's still vague to me on many concrete aspects. e.g. once i hqve a working C++ program, do I have to modify it to make it to be integrated with the API and UI with the mobile app? In the future if there are updates how much more work will be involved regarding the integration with Android and iOS.

If at the beginning I have a good approach to take into consideration of some of the difficulties that are surely coming to me my way, that would save me a lot time and avoid doing things fruitless.

Any suggestions are greatly appreciated. Thanks in advance.

Some to-dos and not-to-dos, Some how to do it and where to find helps.

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