This feature is called branching, and it is currently available in Early Access to all Enterprise customers. We will soon be releasing it for all Growth level customers as well.
Custom css, js, and other code typically exists in your code base. You can also take advantage of cross project imports to reuse the same assets across different environments. This can include images, styles, components. It can also include components that have Custom HTML Embed, where you can write script tags and style tags. Importing projects | Learn Plasmic
You can configure a different ACLS at the team, workspace, and projects levels. You can think of workspaces as folders.
You can just add Google Analytics or any other analytics integration directly to your nextjs or other code base - plasmic will work fine with it added.