You don’t want auto or AV, but don’t let that sound scary.
Lesson 1 w flash is that you have to juggle 2 exposures, and again it’s not scary it’s simple.
Full manual mode, ISO 400, 1/100, f5.6! And how do I know that will be correct rather than using auto?
Because the flash set to ETTL will do the hard bit, and produce the correct amount of light.
From there you can experiment; I’d recommend bouncing the flash off the ceiling, you can also slow the shutter speed to let in more of the ambient light. You con lower the SS very low indeed and move the camera to have the disco lights paint pretty patterns; all manner of useful stuff.
But always remember, sync speed or below for the SS, and you’re balancing the ambient and the flash.
And you’re balancing those whether you’re in a pro studio with 5 mains powered lights (which hopefully obliterate the room lighting) or in full sun where you’re just adding a kiss of full light. And everything in between.