Vessel/artery anatomy of the buttocks? BBLS?

So I recall reading that fat transfers to the buttocks is possibly the most dangerous domestic procedure and there’s a high risk of fat getting into an artery and traveling up to your lungs? Something along those lines. 

My question is exactly WHERE are these vessels and arteries ? Do they lie under the muscles? Right under the skin? Why is this happening to people? I can’t tell by looking at anatomy pics. 

Is the procedure safer for people who already have a decent amount of fat on their butt (aka more surface area) ? 

Thank you for any informative answers