How does PCC not oxidize aldehyde?
The "trick" is in the enviroment, not in the PCC itself.

Alcohols tend to be oxidized to carboxylic acids when they are in aqueous enviroment.

PCC was invented as a workaround to this problem: it works as a reactant in anhydrous enviroment, hence stopping the reaction at the aldheyde stage.

An alternative way to see PCC is to follow its preparation: if you add $\ce{CrO3, HCl}$ and pyridine, PCC is formed.
If you add $\ce{CrO3}$, acid and water, chromic acid is formed.

So, you see that the reagents are similar: the point is in finding at which stage the difference is.

Mechanistically speaking, the difference occurs as follows: in aqueous enviroment, water can form a geminal diol intermediate by attacking the aldheyde. This step adds the second oxygen to the aldheyde that, with further reaction with chromic acid, yelds a carboxylic acid.

When water is not present, this step doesn't occur, and the reaction stops.