found this and it explain it better then I can.
Although shocks and struts are a lot alike in function, they are very different in design. A shock is designed to control spring motion as is the strut, but the strut acts as a suspension part and sometimes replaces other suspension parts. Sometimes used as pivot points for the steering and as an adjustment tool for alignment purposes, struts can absorb side loads where shocks do not.
based on your question..the shock is the tubular black (OEM) thing inside the 'srping' that is attached to the upper and lower control arm. so when you and ppl on here replace the OEM with Bilstein they replace the tubular thing and reuse the spring.
B) Yes. most if not all the members here have replaced theirs with either Bilstein (HD or 5100) or with Ranchos.
C) Not sure why replacing a better part then OEM would void your warranty. If it just void the 'replacement' of future shocks then I say Yes its worth it. If it void other suspension issues then I would ask why and what type of issues will not be cover.
you will noticed your ride will be smoother and better handling once you replace with aftermarket shocks like the bilstein.
Hope that helps.