Lower/upper parts differences

[HiCaliber]

I am still in the process of collecting parts for my first AR-15 build. My question is are all lower/ upper parts (pins and springs, not trigger, etc) more or less the same? My guess is that mil-spec parts are mil-spec parts largely except for perhaps the springs? Would appreciate your input before I waste time looking for particular parts by a certain manufacturer. Thanks.

 

[BERTMAN77MK2]

Just get the known brands and dont worry about it. Quality upper and lower fit is important to me