The PlayMode test called TestHealthBarImageMatchesPlayerHealth() loads the HealthBar scene, gets a reference to the instance object of PlayerManager, which is a component of the PlayerManager GameObject, and invokes the ReduceHealth() method. This method reduces the player's health by 0.1. So, from its starting value of 1.0, it becomes 0.9.
The PlayerManager GameObject also has as a component that's an instance object of the C# HealthBarDisplay script class. This object registers to listen to published events from the player class. It also has a public UI Image variable that has been linked to the UI Image of the health bar filler image in the scene.
When the player's health is reduced to 0.9, it publishes the OnChangeHealth(0.9) event. This event is received by the HealthBarDisplay object instance, which then sets the fillAmount property of the linked health bar filler image in the scene.
The TestHealthBarImageMatchesPlayerHealth()...