Just wanted to know where this code defined in big plasma env. Looking for code location.

Hi!

I think you are looking for /usr/share/plasma/plasmoids/org.kde.plasma.lock_logout if you are looking for the plasmoid.

All our code is on https://invent.kde.org.
Depends what you want to do with: the code for the individual buttons, the icons or the Application Launcher Widget?
There is also more information abut the Application Launcher and possible modifications here: https://userbase.kde.org/Plasma/Kickoff