During chapter 5, you can find the first symbol by completing the reflector trial in Resonance: A Plague Tale Legacy. You don't have to do the trials in any order to open the door, as Sophia and Leni can solve one puzzle before the other. However, the order of the symbols is mostly decided by the drawing found in Sophia's book. Here's how to find the first symbol during chapter 5 in Resonance: A Plague Tale Legacy.
Go through the door that has a circle on top of it to find the first trial.
The door is blocked, so go up the rocks next to the door and climb to the top using the rope.
Insert the sphere into the socket to activate three light sources that will naturally fall onto the three reflectors.
The light reveals the reflectors' colors, which are red, blue, green from left to right. Use any of the lights to remove the dust around the center platform. There are three spots with the same colors you need to shine the lights on.
Although you can change the colors in the setting, I'm going with the original colors, which are Red, Green, and Blue. You can switch back to the original colors if you get confused.
Head to the green mirror and shine the light on the reflector right across.
Next, aim that reflector at the green spot.
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