Does this game have pre-built buildings and rides?

[Sawabunga]

Or does every building/ride/attraction need to be custom built like planet coaster/zoo? I prefer the management side of these games, and don't like to have to spend hours constructing all the buildings.

[AstroTron]

yes

[CMDR Sweeper]

Yes there is, even with decorations so you can focus on management if you really want to.
Or buy most of them, and then build your own single creation you perfect.

[tr!star*]

You also have pre-built blueprints or find even more blueprints in the workshop. You will find everything there, for campaigns look for cheaper ones.