Hi Luiza! (btw next time tag me @marc to make sure I get the notification!)
So, when it comes to the topology, you always want to make sure you maintain square polys (no long rectangles, unless that region won't deform, or is well hidden), and keep the size of your polys consistent across the entire model.
For a limb like an arm where the diameter stays pretty consisent from the shoulder to the wrist, there's no need to add extra loops in there to maintain the shape or size of the faces.
For a bodypart like the torso (hourglass shape) or the leg which starts super wide and ends up very narrow at the ankle, you'll need those extra loops to respect those rules.
The extra loop going around the knee gives me the extra faces I need to maintain a similar density from the lower leg to the upper leg. The loops on the stomach give me enough density for the torso, othewise I would be limited to the loops from the legs - not enough. Same deal with the back, the upper back is wider so it needs more faces to maintain the density, and the lower back end above the butt since I have enough loops coming from the legs to handle the section below it.
All of this because squareish polys deform great, and you want enough density everywhere to deform well, help with the UV process and bake your textures better
Hope this helps?