I didn't even know there is a debug camera. How do you active it?

In ITM it's combination LeftShift+M. Press it during mission then use combination Ctr+5 to center the camera and use NUM PAD keys to move the camera over the map. It's unique feature, which was decisively removed by devs from SBW, to force player it's a tanksim and not a tactics. Everybody knows that true tanksim doesn't has a free camera :)

And subs don't have external views. But how do I find the action? Thanks.

"Free camera" it is hidden feature in the game. I know it was enabled in ITM mod.

37. Q. How to install mod "Free camera"?
A. This feature has been already built in the mod. Assign combination Shift+M in the key setting. See details here.
https://2021-itmtank.forumgamers.net/t1238-free-cam-mod#17180

It is a very nice mod, but I still cannot find the action when I jump in.