50 kilograms is too thin for your height. You'd better keep your current weight and do some exercise, otherwise it will be difficult to get results.
First, increase the absorption of protein in daily life and eat less starchy food. Even if you want to lose weight, it's hard to reduce your intake without protein. Try to sit less, stand more and move more. ...
In addition, exercise is mainly low-intensity and high-intensity, three or four times a week. Aerobic exercise is best done every day (running and skipping rope can lengthen leg muscles), and there is a certain chance that you can grow taller. If you want to practice thigh muscles, you can ride a bike, which will only put pressure on your thighs. Riding too much will thicken your thighs. Don't ride if you don't want to practice, just run.
Abdominal muscles are difficult to practice. The point is to see your perseverance. Do sit-ups every day. You can do less in the first few days, but not less than 100, and then you can add more slowly. When I was at school, I did 240 abdominal muscles every day. Two months ago, I produced four abdominal muscles. After that, I can't practice more than four pieces anyway. Later, I learned that parallel sit-ups
Shoulder training is more bitter, because shoulder width is a bone problem, and bones can't be changed. You can only thicken shoulder muscles through exercise. There are too many muscle groups related to shoulder training, such as trapezius, latissimus dorsi, deltoid, teres major teres minor, rhomboid and so on. Practicing these muscles is basically heavy, pull-ups, rowing, lifting dumbbells and so on. I can also tell you that it is a little easier, but it will take a long time. You take two light dumbbells (the dumbbell I use is 20 kg, so I usually practice with two bottles of mineral water). Do you still remember the chest-expanding exercises in broadcast gymnastics? Just follow that practice. Of course, you can't do only four groups, and you can't do only four groups in one group, depending on your situation, but it can't be less than 20 groups and 6 groups. After practicing this method, you should be able to see the line between the shoulder and the three heads within one month.