Where can i find the “Start Page Container and Responsive Sidebar” source code ?
In many files, depending on which part you’re trying to customize, please tell us.
Which version of SuiteCRM is this? Which theme?
You can start by having a look here:
https://github.com/salesagility/SuiteCRM/blob/master/themes/SuiteP/tpls/_headerModuleList.tpl