This would probably be easier to respond if there was a specific photo you could use as an example.
When you use a flash there are really two different exposures happening in just one shot. That sounds confusing until you think about how the different light sources work.
Some of the "light" in your shot is coming from ambient sources that are providing a continuous source of light. It could be sunlight or it could be electric lights, but they are always on at a steady light level.
When you use a flash, you're adding in more light, but what's different about that light is that it's only a very brief burst of light and then it goes dark. Suppose your flash is "lit" for only a about 1/1000ths of a second... but suppose you are taking a 1/100th second exposure duration. That means the "flash" was on for only 10% of the time that the shutter was open... but the ambient lights were providing light for 100% of the time the shutter was open. Controlling this ratio of flash vs. ambient light allows you to help balance out the lighting in the shot.
Light obeys a law in physics called the "inverse square" law... and I'll use a garden hose with a spray nozzle as an example. Think of the spray nozzle as emitting "drops" of water. If you hold the nozzle directly in front of your face and turn it on then nearly ALL of those drops will blast your face. But if you stand 3 meters back from the spray nozzle, then "most" of the water will miss you and only "some" of the drops will hit your body. The same is true of light... the farther away your light is from your subject, the more the light has a chance to spread out. That means that the amount of light is distributed across a much wider area. Suppose our original area was 1 meter wide by 1 meter tall (1 square meter) but we move the flash back so that it now illuminates an area that is about 2 meters wide by 2 meters tall (4 square meters). While it is now lighting up 4x as much area... each 1 square meter is only getting 1/4 of the total light.
The "inverse square" law says that the intensity of light will drop off based on the "inverse of the square of the distances". In other words if one object is 5' away from the flash and another object is 10' away from the flash then 5^2 = 25 and 100^2 = 100. 100 ÷ 25 = 4. Take the inverse of 4... or 1/4th... and that means the subject 100' away will get exactly 1/4th of the light from the flash as the subject that was 5' away from the light.
This gives you some interesting capabilities once you understand how it works.
You can control what percentage of light is contributed by the flash by adjusting the power output of the flash (and flash exposure compensation or "FEC" does this). You can also control the intensity of flash by varying how close your flash is to your subject. You can boost ambient light by increasing the amount of time that the shutter is open (and that will have no effect on the light from the flash.)
Obviously there are limits to how large or how small the aperture opening in your lens can be (depending on the lens). There are some practical limits to shutter speed as well because the camera has a maximum flash-sync speed (for your 80D that's 1/250th second... you should never use a shorter exposure time unless you are using flashes that support the "high speed sync" feature and have enough power.)
The onboard (pop-up flash) is fairly weak and also doesn't provide flattering light. It is useful as a "fill light" to help eliminate shadows. But since it is a very tiny flash and not particularly powerful (not nearly as powerful as the external flashes) it doesn't make a great primary source of light.
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