Folks tend to forget of the European influence in the United States, you have small pockets of French and Germanic communities and also Hispanic areas, so, you can utilise Spanish, Italian, French and German buildings, for example, structures developed by the many European Rail Simming communities are ideal for any routes that go through these locations on your layouts where appropriate.
There are lots of freeware North American scenery items available, in the library here, road vehicles, houses, warehouses and other commercial blocks, etc, etc, of course, we have been blessed over the years with many freeware routes that have assorted asset packs included with the route, there are a plethora of goodies that can be used on any North American route after collecting all of these layouts, some of them are from 3rd party websites, such as Otto's, but once you have them there is no need for route authors to re-package assets used on their routes, all that is needed is a list in the 'Read Me' of what has been used and who built it, the community can help each other with locating the various assets on routes where everything can be found.
3rd party websites are a hive of useful assets as well, unfortunately, many folks get scared off from these because they are in a foreign language, who would have thought that.....

However, if they went and searched they would find a massive amount of useful content all waiting to be downloaded and installed, and it's all FREE, who could ask for anything better.
For example, this is from a Czech Republic website, you don't need to speak the language, just look at the pictures and click on them to download, there are superb Deciduous and Evergreen 2D and 3D trees and other vegetation that are excellent for many American routes, just give them a try.
http://modely-msts.cz/objektyrw.htmlYou just need Greenery and Smreky 3D, anyone can download them, you just cannot use them in Payware routes, that's all, and they easily install into your game, just like other rwp files.
Cheerz. ex-railwayman.
i7 10700k 3.8GHz Eight Core CPU, Gigabyte Z590 AORUS ELITE AX, 32GB RAM, nVidia RTX3060ti 8GB, WIN10 PRO 64-bit. 10TB HDDs in total.