It looks as though you are using them well. I see you’ve noted clavicles, sometimes the sacral triangle, the spine…etc. In identifying where they are I think that comes with more practice and understanding of the human body. Generally a landmark is going to be an easily identifiable spot on the body in which you can build from or measure from. Usually they are bony spots because those won’t change (where bodily tissues can stretch, fold, squash…). Some of the easier points to identify are clavicles, crest of the hips, head of a the femur, sacral triangle, ridge of the scapula, parts of the rib cage, and 7th cervical. However, sometimes they can also be certain spots on the body (bellybutton, nipple line etc…). These obviously can shift, but I find them to be very nice to do quick measurements from to get a very general idea of things (2 heads to nipples, 3 head down to bellybutton…). How to use them…well they are used for a lot of things. They help you with proportions, they help lay in structure of other tissues, they help with the flow of the pose and can bring a nice balance between flesh (softer) to bony point (sharper) when you draw them. I think for the most part these things become intuitive as you draw more. If you’re unsure about what or where something specific is take the extra time to look. I sometimes find that poking around and identifying something on my body helps me understand where something is better. Find resources with easily identifiable features so you know exactly what you’re seeing. Get good at drawing a basic skeleton for sure. The skeletal proportions in people aren’t really so different. Yes they vary slightly from person to person, but generally they are very similar. So knowing he skeleton (male and female) is super helpful with the figure. Just keep practicing. btw, nice gestures. I really like the “child’s pose” one on the page with the red drawings.
From what I see, I think you have been familiar with body landmarks, and so far it's great. Which landmarks do you have difficulties with? Landmarks are pretty straightforward, so once you know the basics you'll be able to recognize it with ease. I have sketched over one of your drawing to show you the general landmarks we normally use in human body.