When you save SHSH files, they are either on Cydia's server (access by editing your computer's host file) or through Tinyumbrella (locally on your hard drive). They are not saved on the device itself. And, you are using the "brick" term incorrectly. If a phone is brick, there's nothing you can do with it. Just like a brick. The term you should be using is if it's stuck in recovery or DFU mode you can use old saved SHSH files to restore to older firmware(s).
The person who owned the phone previously allowed Cydia to save current and future SHSH files automatically so you don't have to do anything else. Thus, if 4.3 comes out, Cydia will save 4.3 and so on.