Yes, I learn some things at school to do with fitness and exercise, and I learn other things in what I do myself, which is strength training.
Pullups are so good, and if there one pulling exercise to do, that would be it. The other is the Australian row (called the 'Australian row' because you are 'down under' the bar.) An excellent one for posture adjustment, too.
https://www.youtube.com/watch?v=K49EcUzIJEI
Most untrained individuals will be much stronger pulling with arms than pushing, because we usually push with legs, but still, because our habitual posture is leaning over phones, computers, steering wheels etc, most people benefit from 'corrective' exercises that involve pulling the shoulders right back and extending the chest. It is postural, which means position of the bones and the mobility of the joints, mostly a sunken chest, hunched shoulders, forward head position and bent forward/rounded over position of upper spine. Of course it is the muscles which over time pull the bones into that position, so postural correction involves both stretching out the front musculature, mainly pectorals (chest), and strengthening the muscles in back. Australian row does that.
I use the over head squat to correct upper body posture and joint mobility. Nothing tests postural issues like the overhead squat.
https://www.youtube.com/watch?v=RD_vUnqwqqI
Yes, gaining muscle is pretty hard to do, and people interested in gaining muscle size and strength have to eat a high protein diet with a lot of calories. The body will need
at least 0.75g of protein per pound of bodyweight - that's lean bodyweight not including fat - and about 300 calories total daily excess. Most people who start training for musculature and strength fail in their eating.
People who are overweight should not train to gain muscle mass. People need to either lose fat or gain muscle mass. Can't really do both at once.