Thank you @clarkej, I have already read the topic you added, it was very useful for me, but there are still some question.
The reason why I’m asking here is not because I’m lazy to learn, test or experiment. I know that I have to practice and experiment… believe me I have read a lot of posts, guides and manuals here, watched videos, I even already tried to write my own app, which worked. Now I only need an answer to absolutely crucial question for me - if it is even possible to go the way I plan to go, if it makes sense to experiment with it. It can help me (and I not only to me) to learn faster and more effective way, and also save some time. And this is why I’m asking here, because I supposed that is why this forum is here…
May I ask you (or anyone else who is willing to help) only one simple question, if it is possible to move to separate app direct changes to Doctype, made in developer mode - for example change of numbering of Projects? I mean changes, that was not done by Customizing forms or by Custom scripts.