newbie: HOW are controls assigned to joystick?

[falconeye]

As a newbie, I bought <IL-2 sturmovik 1946> +a 16 button <Thrustmaster 16000M > flight stick
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[ IL-2/ CONTROL] menu lists control functions operated from the keyboard.
BUT, the menu also lists essential controls with NO keyboard assignments:
[ Aileron Trim; Elevator Trim; Rudder Trim, Flaps... ]
A mouse click opens a blank "window' next to these control functions.

<< QUESTION 1 >>
How are buttons on the flight stick assigned by using this "window" ?
for example: the < Aileron Trim> control function ?
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the weapons that an airplane carries are activated by a "joybutton"
......weapon 1......joybutton 1.....enter
......weapon 2......joybutton 2...etc, etc
<< QUESTION 2 >>
Are "joybuttons", the buttons on the flight stick?
How are weapons assigned to flight stick buttons?
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After searching I found < HOTAS control >
Power......U slider axis,. . .Aileron....X axis
Elevator...Y axis. . . . . . . .Rudder....Z axis
<< QUESTION 3 >>
Does<HOTAS> refer to the flight sick ?
(what does HOTAS mean?)
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<< QUESTION 4>>>
Finally (for the time being) where in the <IL-2> HELP, instructions,
can answers to questions above be found ?
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MANY Thanks for helping a newbie understand !
falconeye