How does it work?

There are four different packs to choose from and each pack take a different amount of time to change season. It’s a constant shift which is very gradual.

No other vegetation except for leaves shift colors. But perhaps that something for a future update.
After a set amount of time there will be a seasonal change and this is most visible by the leaves shifting colors. It’s very realistic in that sense that it is a gradual shift which happens quite slowly over a longer time. All leaves except for jungle leaves change colors, but that is going to be updated in a future release.