What parts of the body do they take fat from for a BBL?

That is an excellent question. The back, flanks and waist and tummy are the most common areas. So yes, all the areas you mentioned can be used for a BBL. There isn't usually a lot of fat to harvest from the inner thighs and arms, but if needed, these can be used as well. 
I feel that the best fat is the fat that comes from those hard to lose places like the "love handles", for example. In theory once the fat is transferred and is thriving, it will behave just as it did before. This means that areas that do not fluctuate too much during weight loss, are probably the best areas for harvest because they will produce a more steady and predictable result.
Best of luck on your procedure!
Dr. Miguel Mota

Thank you for your question. When planning a Brazilian butt lift, patients can look forward to a double benefit: reducing areas of unwanted fat while gaining a perkier, rounded posterior.  Each procedure must be planned according to the individual needs of patients, harvesting fat from target areas of the torso, thighs or knees and transferring it to create smooth, attractive contours optimized for each person.That said, you definitely have to choose your surgeon correctly and make sure that he/she has the training and experience to meet your goals with absolute safety.  Best is a Board Certified Plastic Surgeon with specialized expertise in this procedure.

Hello, Thank you for your questions.The Brazilian butt lift is a two-part cosmetic surgery procedure that uses both liposuction and fat transfer techniques. Liposuction is normally performed on the abdomen, back, love handles, and/or thighs.The harvested fat is collected and carefully transferred or “grafted” to the buttocks in order to augment, shape, and lift the skin and soft tissues of the gluteal region.The best candidate for a Brazilian butt lift is a patient with a small, square, flat, or sagging buttocks who also has unwanted fat on the abdomen, back, love handles, hips, and/or thighs.Only plastic surgeons certified by the American Board of Plastic Surgery should be considered.

hellomainly  on your abdomen, waist, flanks and back as well.Visit a certified plastic surgeon to accomplish what you wished. Best wishes.
Regards
Dr. Israel Mañon
Certified Plastic Surgeon

In general, BBL includes liposuction of the abdomen, flanks, and back. Any additional areas like, arms, thighs and other areas can be added. 

Hi. We can harvest fat from fat deposits almost all around the body contour. But , yes there is a "limit" to the volume that can be safely extracted, the rule is not to exceed 6 to 7 liters.
It also depends on the hemoglobin level of the patient and the bleeding we observe and calculate during surgery. Is a very delicate procedure even though patients tends to think is the easiest.
International Member of the American Society of Plastic Surgeons
Member of the International Society of Aesthetic Plastic Surgery

You will for sure have fat removed from all around your trunk meaning your abdomen, flanks, and total back but you can also take fat from the arms, inner thighs, outer thighs as well and sometimes we have to take fat from additional areas because there is not enough fat in the trunk. But if you want fat removed from additional areas then that can be performed at the same time as a BBL.

Hello,
Thank you for sharing your question. Usually abdomen, back and flanks. I suggest you get a consultation with a board certified plastic surgeon for more information.
Kind regards,
Dr. Roberto Guerrero, MD