How to electric motors work?
Brushed motors can run without a controller, higher voltage will allow them to run faster, a BLDC motor has to be used with a controller and that determines the speed, while the voltage determins the maximum speed possible.
By using a BMS and also setting a max current limit in the BLDC controller.
Depends on the BLDC controller, you set the max power limits in it.
Voltage and current dont work aganist each other, you can have both if your BLDC is correctly matched to your battery and controller and big enough.
It has a gearbox, just not with adjustable ratio, electric motors have insane low speed torque.

There is a lot to learn about BLDC motors, so i would recommend getting a small motor and a good controller and trying different scenarios to better understand how to properly control them.