What can I do to achieve a more defined jaw? 28 y/o (photos)

I would recommend doing minimally invasive skin tightening with radiofrequency like ThermiTight to tighten your skin. You do not appear to have fat that needs to be removed.

Your skin is good, you just have a little fat under your chin. To define the jaw, the best thing would be to liposuction all under the neck, from ear to ear, at the same time take a little fat from around the belly button and transfer about 4cc to the chin. The combination of this will give an appearance of a more defined jaw. I would not do any transfer or fillers at the jaw angles, although that can define the jaw further, you do not want a square face, and fullness on the bottom of the face.

I encourage you to visit with a board certified plastic surgeon for a consultation. If there is excess fatty tissue then jawline liposuction should help improve the contours. You might also consider FaceTite to tighten the skin and/or Morpheus8 which addresses skin tone and texture.