external with IG menu

I used to use this hack that was external but when I was in csgo I could press Insert and I would have options like an internal how is this possible?

External only means that it doesnt take advantegae of the .exe file
A hack can be external and still have a menu ingame, you can see the menu more as some kind of overlay, or layer, which just will be put above the .exe
Basically like Photoshop layers

This is actually possible if the cheat asked of you to be in "Full screen windowed mode", so uhm, yeah, just like the guy above me said, it can be easily done as an overlay just because youre in "windowed mode".