There are primarily two things to consider.
- The light coming from your flash.
- The light coming from whatever you are photographing(your subject).
The light coming from your flash is an illuminating or transmitting light source.
The light coming from your subject is a reflective light source and is being illuminated by by whatever transmitting light source is available at the moment (available light).
All your flash does is add light to your subject.
You now need to consider the exposure for your flash and the exposure for available light separately.
Your flash exposure can be equal to your available light exposure.
Your flash exposure can be darker than your available light exposure. (most photographers call this “fill flash”)
Your flash exposure can be brighter than your available light exposure.
It doesn’t matter whether you are using the pop-up flash on your camera or a separate flash unit. When you begin to see flash and available light as separate components to your photograph, you can begin to manipulate their relationship in terms of relative brightness/exposure to each other and create thoughtfully composed photographs by using whatever flash you have.