What is the flight controller? If always thought that was the remote with the throttle
No, that's your radio. The FC (flight controller) is what accepts the signals from the RX (receiver) that's sent to it by the radio,
it then translates those signals (your input commands) into different signals that then are sent to the ESCs (Electronic Speed Controllers)
which control the motors. In summary, the radio (in your hands) transmits to the RX, that transmits to the FC that transmits
to the ESCs.
Not to confuse the issue, but if you have RSSI (Received Signal Strength Indication) then the RX also acts like a TX and the
radio (which is usually only a TX) also becomes a receiver ! ... This is typically found only on "Hobby Grade" quads and not on "Toys".
It is extremely useful in that this enables you to monitor conditions on the quad like it's battery voltage, how well it's picking up your
signal, current drawn, etc, etc.
In an FPV (First Person View) setup there's also a Camera, VTX (Video Transmitter), VRX (Video Receiver) and the display (screen or goggles).
The usual setup goes something like this: The battery wires go to a PDB (Power Distribution Board) which takes the varying battery
voltage then filters and stabilizes it at 5VDC and 12VDC. These voltages then feed the RX,FC,VTX, camera, etc.
As far as identifying which PCB (Printed Circuit Board) is which, it goes (excluding power leads) like this: The RX has the antenna wire(s)
coming off of it. The RX is only connected to the FC. The FC is connected to both the RX and the ESCs. The ESCs are connected to both
the FC and the battery.