Thank you for the kind words!
1. It's reaching 25 M population, the tutorial tell you as such, you can re-enable the tutorial in the settings before you start a new game.
2. It's the happiness meter, above 0.5 the population increases, below it decreases. As I changed the balancing of the game, it became nearly obsolete since the available surplus power limits the growth and it is hard to overshoot the production.
3.Ye boi