We need to get rid of that triangle because of bad subdivision. Use cut for creating line that correspond with fold line and make doted line edge invisible (right picture)
Extrusion was quite fast but the buttocks are not round enough yet.
Two series of Cut Edges allow to round all that.
Position the vertical edges regularly around the back and side part of the thigh and the hips.
To mark the fold gluteus, add the marked edges and turn them visible (the dotted edges). Make invisible the others to have the same aspect as on the image on right side.
Aspect of the grid of the buttocks seen from back and left.
The subdivided mesh, iteration 2 in Meshsmooth.
Extrusions for the top of the bust.
Two extrusions for the start of the neck.
While preparing for modeling of arm, simplify the for faces into one polygons (Weld Target).
Insert the edges (see picture on left) for a better round-off of the arm.