Do temporary triggers all happen at once or individually?

Each permanent with Temporary generates its own separate trigger. They all go off at the same time (the start of your Beginning Phase), but they are placed on the same chain as individual items, not merged into one.

If you control multiple Temporary units, you choose the order they go on the chain (Rule 383.3.d / FAQ #7341). Because the chain resolves last-in-first-out, the trigger you put on last resolves first — so order them in reverse of how you want them to die if you care about Deathknell sequencing or Reaction windows.

If both you and your opponent have Temporary units, the turn player's triggers go on the chain first, then the opponent's on top — meaning the opponent's units' triggers resolve first (FAQ #2439).