Need help getting documentation of Jenkins
When creating the Jenkinsfile for a pipeline (using groovy) how is the code structure made? Where can I find some examples starting from the easiest to the complex ones?

The files typically ending in *.jenkinsfile can be made in any text editor or IDE. For examples the best place to look is the Pipeline Examples page. There is also a github repo with more examples.

What are the actual differences between stages and nodes on Jenkinks file?

Nodes - are physical machines or virtual machines that run Jenkins as worker. They can be different architectures.
Stages - These are best thought of as the recipe:

Mix the ingredients
Add water
Mix again
Bake

Each stage is an action that Jenkins needs to take.

Where can I learn how to make pipelines from scratch (but not just copy/paste from somewhere, understanding what I'm doing it), i.e.: I want to run a code on the server where I have my Jenkins master every time a fellow coder make a successfully git push

The first two links I gave are the best way of starting but it's purely subjective. I'd suggest starting simple and then building up from there. In the document you referenced there is also a section on Pipelines which might be the missing step for you.