Star Spring

It depends on the scenario you're asking about, but the core ruling: Star Spring triggers once per player each turn, not once total — the first time each player plays a non-token unit there, that player may move another unit they control at Star Spring back to its base.

Three things people get wrong: only an actual play counts (moving a unit in does not use up or trigger the first-time condition), tokens never trigger it, and a unit that enters via a replacement effect never "enters here" — e.g. Baron Nashor played to Star Spring goes to Baron Pit instead, so no trigger (Rule 369.3, FAQ #12852).

The player who played the unit controls the trigger and picks which of their other units moves — not the unit that just arrived. What's the specific board state you're working with?