When two jetbridges are used, the bridge used depends on the deck your seat is on.
When three jetbridges are used:
Door 1L is for First.
Door 2L is for remaining lower deck passengers.
Upper Deck bridge for all upper deck passengers.
I don't think any airline boards A380 passengers such that they need to use the front or rear stairs.
Mirrors my experiences on a dozen or so flights on it to/from HKG.
I have had single door disembarking (from 1L) but never single door boarding.