Idk how that works in godot but with other engines, there are libraries or extensions that can handle that or if you really want to go the "painful but classic way", just play a frame, capture it as a screenshot then go onto the next frame until the game ends then make a video out out it. Idk how audio would get taken into account with this method tho. honnestly if the game was made in Unity (that's the only thing i have experience modding) i would have liked to make a mod that exports replays in video