Window Air Conditioner Support Bracket Heavy Duty, Up to 165 lbs, Fits Up to 24K btu A/C Unit

What keeps the support brackets in place

How to install? the arm that goes on the outside wall wont stay in position because the screws cant get tight enough.

is it pre assemble?

My unit which is also a dehumidifier needs to be angled down slightly in the back to release the condensation, how can I do that with this device?