Stretch marks are the best sign of progress!!! Rare when it happens, takes radical growth that doesn't happen that often during a training career.
The areas mentioned are often hidden or on the inside side of the body, which is less obvious: inside of thighs, underside of upper arms, underside of lats near armpits. Literally areas that ar stretched during and after exercise and getting pumped. Some of the more visible areas are the outside of the armpits around the chest/delt tie-in, and the bis.
With time they fade back in to almost the skin color, the only difference being a different texture that's not that obvious. I have to really look to find em they've blended in so much.
I don't think creams really help, it's genetic.