You need to spend hive/sim tokens to buy assets/building to play the game = build your city.
Then you earning hive and sim tokens and it's up to you if you going to take your profits or build up your city and go higher in ranking for more rewards.
https://hive.blog/dcity/@ironshield/introducing-dcity-io-a-fun-way-to-earn-sim-tokens-and-hive