To understand what drives policy development, we need to think about the geographic scales of influence on any given body which may develop and implement policy. A useful framework for this is to think about vertical and horizontal policy diffusion.
Both types of policy diffusion are critical to addressing the global climate challenges we face. Think back to the Clean Power Plan - while inherently a federal policy, it offered states wide-ranging flexibility in how they'd set and meet their targets, including the opportunity to collaborate regionally. This illustrates both vertical and horizontal policy diffusion. The Paris Agreement is another example - it's an international agreement with nationally determined contributions that very likely would require integration at the state and local scales as well. But what if one level of governance isn't as active as others? That's a bit of what we were seeing during the Trump Administration. While the federal government in the U.S. wasn't actively seeking to address climate change through federal policy measures, states, municipalities, and businesses were continuing to work in this space and were learning from what their counterparts were doing - a strong example of horizontal diffusion.
One common way that policies are actively diffused is through publishing best practices, case studies, and other resources. Not only can you save time by modifying an existing policy instead of creating it from scratch, but you can also evaluate the effectiveness of the policy where it was originally applied. See e.g. the Resource Library [2] for the Global Covenant of Mayors for Climate and Energy [3], which is one of the organization identified in Lesson 5. The Horizontal and Vertical Reinforcement in Global Climate Governance article is a good read if you're interested in understanding this better.