Drained battery, then cooling fan turns on?

Hello, with the fans on constantly that is a sign that the engine computer is not communicating. It's a default strategy when the fans stay on like that. Does the Check engine light come on with just the key in the on position?
And what size engine does the vehicle have, so we can look up the correct wiring diagrams.

Okay, with no check engine light coming on the PCM is not being fully powered up, these are the fuses to check first, the battery did not get hooked by backwards by any chance, did it? There is a Battery current sensor on the Negative battery post, it did not get unbolted from the actual cable connector that goes on the negative battery post? On some of these battery current sensors, once they are unbolted from the cable, that nut and bolt break on some of these and are not reusable. It's designed like that; the battery current sensor also has a Lin bus communication wire that comes off of it that goes to the PCM. I'll try to find a picture of it.

The torque spec for the negative terminal with a current sensor on it is only 3 to 4 ft lbs, which is very little. Over tightening it can damage the current sensor internal circuit board.
When the battery is being recharged in the vehicle the ground clamp from the charger should be grounded to the vehicle body and not to the battery negative terminal.

Do you have a test light to go through all the fuses to see which ones have power? If any have power, we can see where it gets power from and work backwards from there. Also do you have a multimeter to see what the voltage at the battery is. Or is the battery dead already?
You pulled the fuse to get the fan to stop running?
It sounds like there is a main fuse in the engine compartment that might be blown, unless the battery is completely dead, if it is there might be a large parasitic draw on the battery that's just draining it extremely fast
This is a diagram of one of the Multi Fuses, you can see they are in a plastic case and can be difficult to see if one fuse is blown inside them, usually there will be a burn mark on the inside of the plastic where the fuse burned, but sometimes the entire multi Fuse can be difficult to get out and the cover doesn't usually come off them.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-use-a-voltmeter