Basically the question is: Can a user access the installed Add-on's code in any way like e.g we can access through Tools Menu -> Script Editor(on SpreadSheets) while on development & pushed through clasp CLI?
Negative. Users who have installed the Add-On do not get to see the Add-On code. If they open a Apps Script editor instance from an editor in which an installed add-on is running, the user will get a blank Apps Script project template to work on, not the Add-On code.
They will however see the names of the functions executed in their Apps Script dashboard.