What specifically changes in the Windows environment when opening the control alt delete menu?

Solution 1:

The Ctrl + Alt + Delete combination, more formally known as the Secure Attention Sequence, brings up the Windows Security dialog on a Secure Desktop. This is a completely separate desktop from the one all other applications run on (\Sessions\1\Windows\WinSta0\Winlogon vs \Sessions\1\Windows\WinSta0\Default)¹. Further information is available in Microsoft documentation.

This behaviour has existed since the Secure Desktop was introduced in Windows Vista. A similar process is used for UAC elevation prompts by default.

The desktop switch can cause issues with some applications, especially those using 3D graphics. In the early days of Vista, it was particularly slow due to how it interacted with early WDDM graphics drivers.

As an alternative, if your requirement is to open the Task Manager, you can do so with Ctrl + Shift + Esc or by right-clicking on the taskbar. This will not cause a desktop switch.

¹ From Chapter 7 of Windows Internals 7th Ed Part 1