So there’s a few ways. One of which, you can head over to www.nedesigns.com and download some workbench parks. Once you load the park in-game, all the custom objects and rides they obtain will be automatically added to OpenRCT2. You can also do this with any of the parks over there, I’d suggest checking out any of the “Head-2-Head 8” parks, as they have a ton of custom scenery and rides that will get you a really good set of objects to play with
A second way is to put any manually downloaded objects into your OpenRCT2/objects folder. You can do this with any of the individual objects