Unlimited money is the most searched-for option in Cities: Skylines. When it is on, your budget never goes down and you can build anything you want. It is the right choice for players who see city planning as a creative pursuit, but for those who see managing the economy as the real challenge, it changes the experience completely.
Most codes here need cheats turned on first. See the section on how to open the console.
Content Manager > Mods
Your budget never goes down, and you can build anything you want. It is the game's own built-in option, so no mod download is needed.