well the body does make protein, technically amino acids but i think thats what you meant.
there are essential amino acids that the body doesnt make but has to be taken from external means. these are the ones that help rebuild new cells and replace the worn out amino acids/proteins.
and anyways, to GAIN mass (fat, muscle whatever) you have to eat at a caloric surplus (ie to lose a lb of fat you need to drop 3500 cals from your diet, to gain a lb of muscle (or fat) you need to increase your intake by 3500 cals)
theres 2 problems, most people here are trying to lose weight. not gain it (caloric deficit); and those who are trying to gain it, cant spot gain just like they cant spot reduce (not to mention during 'bulking' you can only hope most of what you gain is muscle not fat).
doing squats and lunges will definitly lift, tone and tighten, but to go from virtually no butt to an onion butt...its going to take ALOT more than slurping down a cheap protein powder (which might be crap for your body) and doing some lunges and expecting a big rear.