3. I have a website that is accessible through multiple domains, and each domain may have its own configuration and unique traffic. In Google Optimize, I used separate containers for each domain and ran tests independently. What are the best practices in GrowthBook for conducting the same experiments on multiple domains, especially when each domain has a separate GA4 account and its own analytics system?

You can do this a number of ways. Probably easiest is with one project per site. You can then tie the metrics and data sources per project

One of the biggest drawbacks in Optimize was the absence of a feature to duplicate experiments from one container to another. I had to create experiments from scratch whenever I wanted to test something across multiple markets. How does this work when using projects and environments in GrowthBook?

we have a way to copy features

Do these copy apply in both of the ways you mentioned above?

We don't currently have a way to duplicate an entire environment or an entire project, but you can copy features.