Gimme Cloud Shadows is the best way to create shadows moving across your game world for Unity (Built-In + URP).
It is simple to use by default, but has some very advanced features as well to create the effect that you want. This includes an additive mode, cloud shearing with the direction of the main light, procedural texture generator etc.
Not only that, there are three different techniques implemented to render the shadows into your scene, so that they are easy to integrate even for existing projects that are further along.
In addition to the shadows, there are now Sun Shafts as well!
Also included are a lot of tools for creating or modifying 3D Textures, including a noise tool for making animated shadows.