Players will enter the following states once the Season finishes:
When the Season comes to an official close, the servers will undergo an extended maintenance period to determine the final winners. You will not be able to join that world for the time being, but can still play on other servers. Once the winners are determined, the game will then enter a Preparation Period.
You can find out your Season performance and collect rewards that were given to you. Mechanics like combat and city production are restricted at this time. Players are allowed to make final preparations like drawing packs, collecting prizes, combining monsters, and coordinating with your allies.
New Season Start
When the new Season starts, some aspects of the game will be carried over while others are reset. Generally speaking, game progression like your city, land control, and monster's levels will be reset. Other game elements like purchased currency, game booster items, and the monster's skill levels will be carried over. This can vary by Season so you should check for any announcements for more details.