BBL different approaches: In that you put fat under the muscle versus not? Is this true?

Theoretically, intramuscular fat injections provide a better recipient site for fat graft survival than subcutaneous injections due to the better vascularity of muscle compared to fat.  However, the muscle also has larger arteries and veins, so complications such as hematoma and even more serious implications such as fat emboli are likely to be more common.

Far more important than the technique is the skill and experience of your plastic surgeon. Choose your surgeon rather than the technique and let them explain why one technique may be better than another.  Please visit my page on finding the right doctor at drnichter.com on some suggestions on finding the most qualified Plastic Surgeon for a BBL Always insist on a board certified plastic surgeon. 
Candidates for Buttock Augmentation surgery desire an enhancement in the buttock area to offer a more balanced, defined or shapelier contour to the body. There appears to be two different groups of patients seeking the procedure. One group of patients is younger and has goals of transforming their bodies into more curvaceous figures, like popular Hollywood stars, J. Lo or Beyonce Knowles. The other group of patients is women that have aged and would like to restore a more youthful backside with buttock fullness to replace now sagging areas. Men have also been requesting buttock implants as well.

Techniques will vary especially with regards to BBL. Some grafting planes are without a doubt more dangerous than others.
Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
At the recent American Society of Plastic Surgeons Hot Topics, data from AAAASF (ambulatory surgical certifying body) was presented, suggesting that BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund is convening a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability)and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas, aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

The answer is quite simple. Injecting fat into the muscle for Brazilian Butt Lift increases the risk of fat emboli and DEATH. A good plastic surgeon can give a patient a FANTASTIC result on the first try without subjecting a patient to this risk by limiting fat injection to above the gluteal muscles. If you want more volume after your first surgery, it is better to come back for additional fat transfer than to take the increased risk of death from a fat emboli due to having fat injected into the muscle. As your normal butt anatomy is right now, your fat is ABOVE the muscle, not within it. 

I usually place the fat in the subcutaneous fatty layer and maybe in the superficial muscle layers. Some place it deeper but may be prone to higher risks of complications.

BBL procedures can be done somewhat differently doctor to doctor.  But, more importantly, they are done differently Patient to Patient!  Not every BBL is the same, depending on where each person is starting and what is their goal.  A small amount of augmentation may be performed above the muscle only.  As the volume to be grafted increases augmenting into the muscle is a better choice to ensure that the fat graft "takes" as well as possible.  Hope that helps, but your best option is to schedule a consultation with one or more plastic surgeons to get an evaluation and opinion.