To finish Metro Exodus 100% and unlock the Platinum Card Trophy, you must unlock all of trophies/achievements. We clarify the task to you:
Grab a demon with fire.
An overview of all trophies and achievements can be located here.
Kill a demon with fire
To receive the Firebird Trophy, you must kill a demon with fire. In Chapter 3 (Volga), a demon is hidden in a nest that you can easily attack and kill with Molotov cocktails.