The RAT (Ram Air Turbine) supplies ‘just’ enough electrical and hydraulic power to power the flight controls in the event of a total systems failure. It’s a ‘get you home’ device that has, as far as know, never been used in anger!
It’s a bad day in Dodge if you’re relying on the RAT!!!
The RAT is used as noted by the post by @Yuan_Tugo. The RAT does not power the APU as you noted “we have an APU and stuff like that for the RAT to power.”
If you have the APU you can use that for power and hydraulic power - albeit you still need an engine for some forward thrust hopefully. With no engines basically you better find somewhere to glide to for a landing, but at least you will have electrical power and hydraulics from the APU.
If you have no APU or engines that’s when you are down to the RAT to provide very limited electrical and hydraulic power.
I can see maybe adding something where when you run out of fuel it simulates the RAT. I don’t feel we need an entire new animation, otherwise people will purposely takeoff with not enough fuel just to see it, then they will be spamming ATC with request to emergency land.