You float in space and destroy what is around you, but the scoring makes you think before you fire. A green block adds five to your score, a white one takes three away, and other colours buy or burn seconds on the clock. Shoot carelessly and you run out of both points and time.
The spawning system
The interesting piece is what keeps the field full. The spawner runs on three functions. Start fires once and decides when the next object appears, Update runs every second and calls the spawn when the timer says so, and the spawn function picks random x, y, and z coordinates, picks a prefab, and drops it in. Randomising both type and position is what stops the game settling into a pattern you can memorise.
Game manager: the start and update functions.
The object spawning script.