Really varies. Depends on the airline and the airport - some airport/airline combos sometimes have special features like boarding directly from lounges, double decker jetways (the A380 almost requires this), a few have car service for First (FRA, MUC, ZRH for LH plus CDG for AF) etc...

Generally boarding is a first, business, economy order (often but not always with a "bypass" if a First/Business pax comes up after the initial call) with status passengers slotted in somewhere (1st + top tier). But if an airline wants to have a general cattle call to fill up an A380, that is up to them.