LibreOffice Online

hello all,

I am very sorry for this question as i think it may be easy for many but I
spend a lot of time searching and I didn't reach my goal .

if I click on button in a menubar of writer for example and click on insert
then sections i know that sections is added to Control.Menubar.js as
uno:.onu file .

I want to know how this dialog appear and understand how uno element handle
this click and where is the files that responsible for showing this dialog
and make changes in writer writting area .

I am very sorry if the question is easy but I need to understand it .

Thanks in advance