Hi.
In order to offer as much modifiers as possible regardless the keyboard being used, the NoMachine app implements an extended keyboard bar. You can see it on top of your virtual keyboard or, if you’re using a physical keyboard, you can open it with three fingers tap or by tapping the Keyboard icon in the side toolbar. In the extended keyboard bar you’ll find Control, Esc, and other useful modifers and keys.
As for the request to map keyboard modifiers depending from the remote OS, you may be interested in the following Feature Request:
https://www.nomachine.com/FR10M02977