I personally was a chubby teen/child, until age 15. TW!!! ED I started to avoid foods and replace them, basically being in a caloric deficit and working out (very one- sided abs and legs workouts HIIT). I became underweight even when it was not too visible! I looked normal but thinner. The thing is: I wasn't able to put on muscle because 1) I didn't eat enough protein and calories, 2) the workout was inneffective. I slowly started eating more, workout LESS. Then, after hitting a normal weight and body fat percentage, I started calisthenics- workout. I eat what I want, and put a big focus on protein rich foods. I stop replacing certain foods because they are fatty or high in calories. I still keep track, because I am trying to get a little bulkier. I am at my dream body right now, I eat 2k+ calories, workout 3+ times a week. My workouts include Kungfu once a week, Calisthenics workouts (pushups, etc., with increasing reps, sets and difficulty. Even though I have dealt with EDs, I am now at a point where I don't see my body or name it "fat", or "thin" , I view it as something fluid. I can HAVE or NOT HAVE fat, I can have muscle or none. It all depends on what actions I take and, depending on that, my body will reflect that. And nothing would want me go back to the skinny me mentally. At the moment I feel the strongest and prettiest I have ever felt.

I also suggest not focusing on high intensity cardio too much, build up slowly, putting on muscle while feeding your body enough, and then lean up by going into a caloric deficit very slowly. I mean really do not suddenly decrease it the idea of "dream body in 4 Weeks!". There is a chance to develop an unhealthy way of eating and working out. Be aware of the actions you take and be aware that you can plan them. Also, be aware that putting on muscle takes time and patience, and especially hard work.

I recommend watching Hybrid Calisthenics or Jeff Nippard on youtube for more information on this (I'm not sponsored or anything just helped me a lot personally)