The available developments are all public information, chosen randomly at the beginning of the game.
You can buy a development (instead of techs) any time you take the Research action.
Each development may be purchased only once, by a single player. Most developments are instant, one-time effects (e.g., the +5/+5 for +12 developments, or the +3 VP Ancient Monument), but some put tokens on the board and remain in play indefinitely. Shellworld, Warp Portal, and Trade Fleet all stick around for the rest of the game.