how to do ALT+ TAB for main system while in VBOX?

My solution: VirtualBox: File -> Preferences -> Input -> uncheck "Auto Capture Keyboard"

1. Enter into VirtualBox

2. Press the Host key.

   • The default Host key is the right Ctrl key.
   • On macOS, the default Host key is the right Command key.
   • The previous version of this answer listed the Alt key.

3. NowALT + TABshould work.

Regarding the Host key from VirtualBox Manual Ch. 1

To return ownership of keyboard and mouse to your host operating system, VirtualBox reserves a special key on your keyboard for itself: the "host key". By default, this is the right Control key on your keyboard; on a Mac host, the default host key is the left Command key. You can change this default in the VirtualBox Global Settings, see Section 1.15, “Global Settings”. In any case, the current setting for the host key is always displayed at the bottom right of your VM window, should you have forgotten about it

Two part solution:

(1) My solution: VirtualBox: File -> Preferences -> Input -> uncheck Auto Capture Keyboard (taken from @baronbaleron above)

(2) Remap Cycle through open windows to ctrl+tab. In Linux Mint (a derivative of Ubuntu): Start menu -> Keyboard -> Shortcuts -> General -> Cycle through open windows -> change binding to ctrl + tab

Because I use my left hand to do Alt+Tab, I personally set my Host key to be Left Ctrl. This makes it much more easy to switch if you have your right hand sitting on the mouse: press Left Ctrl then Alt+Tab.