Borer: in malacha of borer one can seperate the good from bad; but there is another din that one cannot seperate things into categories (ex: different sized plates)–therefore why can you take good from bad? You are in fact separating mixtures? and
Based on the above why can’t I take meat from a stew and put it in the fridge? Why is this a problem of seprating?
Fridge: If by opening the door to a fridge the fan definatley shuts off until one closes the door again and then turns back on is this permissible?
The idea of taking the good from the bad is that the deed one does is not an act of separation, but rather an act of eating. By separating the good from the bad, with hands/fork, and with the intention of eating immediately, the act is clearly not an act of sorting, but rather an act of eating. It is permitted to eat, and therefore there is no prohibition involved.
This is not the same as an act of sorting, such as taking meat from a stew and putting it in the fridge, which lacks the condition of doing the act for the purpose of immediate consumption. The fact that the meat is not being eaten right away demonstrates that this is not an act of eating, but an act of sorting, which is prohibited under borer.
Fridge: If opening the door shuts off the fan immediately, as a direct consequence of opening the door, and closing it immediately turns on the fan, it would be a problem to use the fridge on Shabbos, unless there is some way to shut off the fan mechanism.