Check out this 2019 tutorial on making a Snow Shader in Unity using Shader Graph.
Back in 2019, a YouTuber Brackeys, who used to create various Unity-related tutorials, released a great guide on using Unity's Shader Graph to create snow. Brackeys explained how to apply snow to a single object and then expand the shader and apply it to an entire environment. The important thing is to use Unity 2019.1.2 or later and make sure your project is set up to use the Lightweight Render Pipeline, otherwise, the proposed workflow might not work.
Even though Brackeys doesn't create guides and tutorials anymore, the existing ones can still be found on the channel. So if you are into Unity, we highly recommend checking out Brackeys YouTube channel to learn more about the engine's capabilities.