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Normally, when you import the "Bridges in seconds" Asset, a Welcome Window is displayed. It detects that you are running the Universal Render Pipeline (URP) and displays a message associated with a button to import a specific package that contains the materials and prefabs that fit the URP. If it does not happen or if you have not clicked on the button, most of the materials will be displayed with a dull color. Don't worry ! You only have to navigate to the project folder "Assets/3DWorldInSeconds/Bridges/UniversalRenderingPipeline", then double-click on the URP_Specific_Pack.unitypackage. It will open the import window.
Bridges prefabs are so simple to create that there is no artistic added value in doing so. You can use your prefabs in your terrains to integrate them in your games but not distribute them in the Asset Store.