Additionally, if you are interested in following the execution of the Matlab code step by step, you should learn how to use the Matlab debugger. Watch an online tutorial, then place breakpoints in the code (click on the line you want to explore from) and then execute the code line by line, explore the content of the variables, etc.
The difficult part is to know from which function to start from. If you click on a button or a menu that has a label, search the Brainstorm code for the corresponding string (Shift+Ctrl+F, search subfolders), place a breakpoint at the beginning of the callback function associated to this button, then click on the button. The debugger should stop at the beginning of the callback.