All mods and assets can be installed locally on Cities: Skylines, and local versions can co-exist with Steam versions just fine. Local just means that for a Steam Workshop user, they won't show up as a subscription, and won't be automatically updated or deleted should the author change anything. For Epic games or Origin versions of CS, this will be the only way to use thirdparty content.
Now these instructions are just for the Windows version, though there should be a method for Mac users, console users will be left out. Note that <username> is the placeholder for your Windows login name. Since Windows hides certain directories by default, it's easiest to copy the entire path and replace the username part, then paste it into the address bar, rather than trying to navigate there visually with Explorer and not seeing the relevant directories. The instructions below will assume Windows was installed on drive C: and not a secondary drive.
For general assets like buildings, roads, interchanges, trees and so on, the place to extract them is to their own sub-directory under: C:\Users\<username>\AppData\Local\Colossal Order\Cities_Skylines\Addons\Assets
For all maps, you likely won't need unique sub-directories as there aren't going to be frequent name overlaps, it is still good practice to make one for them under: C:\Users\<username>\AppData\Local\Colossal Order\Cities_Skylines\Maps
Any mods can also be installed locally, especially useful for non-Steam versions of the game, and most of them are available through thirdparty sites (like citiesmods.com and smods.ru), but is not relevant for any downloadable asset posted herein.