The ODROID-C4 provides 40-pin dual row expansion header “J2”.
The location and pinout of these connectors are illustrated below. (Board Revision 1.0 : 2020/01/29)
All signals on the expansion headers are 3.3V except for the analog input signal.

Note that I2C-1(Pin 27/28) is shared with add-on board RTC chip PCF8563.
So the GPIO feature on Pin 27/28 is disabled by default until you change Device Tree file.

Two 5V / 3.3V Power Out ports each in 40 Pin expansion connectors. 5V Out is pin number 2 and 4 and 3.3V Out is pin number 1 and 17.
They each be connected to the DC-DC converter directly.
Measured at 5V(U25) and 3.3V(U27) DC-DC out on the board how much the power consumption they used during the OS Ubuntu Mate / Android is booting, idle, and until power off.

* Measurement environment

• OS : Hardkernel official Image - Ubuntu Mate and Android
• Clean Images - No installed utils and only connected HDMI, Ethernet 1Gb, and USB-Uart Kit.
• Measurement tools : Digital multi-meter, Smart power 3

• Peak(Max) Current measure by Digital multi-meter
  • Ubuntu Mate : 0.102A
  • Android : 0.099A

• Peak(Max) Current measure by Digital multi-meter
  • Ubuntu Mate : 0.299A
  • Android : 0.303A

• Power consumption measured by Smart power 3 ( It is calculated to Ampare at peak)
  • Ubuntu Mate : Roughly 0.3A
  • Android : Roughly 0.3A

There is C4 Power Hierarchy on page 4 in the schematics that you could see the whole power structure and roughly guess power consumption.
The first revision of C4 has been released with 5V/3A output( U25 DC-DC in the schematics ). but shortly afterward, because of a shortage of material, it has replaced with 5V/2A.
Refer to the C4 PCB revision history.

5V/3.3V power seems not to be much consumed by the above environment.
The 12V/2A(24 watts) Main power consumes roughly 3.5 watts for the whole C4 components. It could be ignored in order to derive the result since their DC-DCs are 10 watts for 5V and 6.6 watts for 3.3V.

If you want to use a 5V/3.3V power source via an expansion pin, the measurement could say ( without anything connected & installed any apps on the ODROID-C4 that is the above environment )

• Theoretically calculated 5V out in the expansion pin is :
  It has 5V/2A 10W DC-DC. With the above environment, the 5V expansion pin has room to supply power 9.5W. It could be supplied a current 1.9A via the 5V expansion pin with NOT anything connected at the pins.
• 3.3V out in the expansion pin is :
  It has 3.3V/2A 6.6W DC-DC. With the above environment, the 3.3V output pin has room to supply power 5.6W. It could be supplied a current 1.69A via the 3.3V expansion pin with NOT anything connected at the pins.

But, there is something we have to consider the PCB routing length and width, the wire's length and its followed impedance and inductance, etc…
so, have done an experiment to connect the loader machine to check how much the current can supply to the external via expansion pins 5V and 3.3V.

• 5V out in the expansion pin : Pin 2 and 4 are connected simultaneously to 5V Power and pin 6 to the Ground.
  When 1.5A load is loading on the pins by the loader machine, it dropped from 5.2V to 4.9V. The load could be increased a bit more. but it is not worth it.
  

• 3.3V out in the expansion pin : Pin 1 and 17 are connected simultaneously to 3.3V Power and pin 6 to the Ground.
  When 1A load is loading on the pins by the loader machine, it dropped from 3.29V to 3.01V.
  When 0.6A load is loading on the pins by the loader machine, it dropped from 3.29V to 3.14V. The 0.6A load is inside the 5% margin.

But adding utilities or apps is able to make it have higher power consumption than your thought.

We can NOT guarantee how much power supply via the 5V / 3.3V in the expansion pin. It really depends on your use case of C4.
For example, It depends on how many USB devices are connected which consume 5V current, or how many processes running which could be consumed 3.3V current or something,
There are so many unexpected situations on their power consumption.