If I create a Copilot in Copilot Studio, how can I create different branches for that Copilot? For example, a development branch or a testing branch, etc.
ALM in Power Platform doesn't have a native concept of branches, but it is possible to (1) work with solutions and multiple environments, so copilots that are ready for QA get pushed to the test environment and then finally to prod, based on your desired pipeline, and - (2) export solutions or copilot templates and store those in a source control repository.
#2 will allow you to manage different branches, although those branches won't be exactly recognized by Power Platform.
Here is some more info on ALM in the Power Platform: Application lifecycle management (ALM) basics with Microsoft Power Platform - Power Platform | Microsoft Learn
Does this help?