From the blog post here: https://www.mendix.com/blog/extensions-the-natural-evolution-of-the-developer-experience/ you can see that the extensions are like modules and are specific for a project:

"You will be able to upload and share the Extensions you create on the Mendix Marketplace. That’s also where you’ll find extensions created by other developers to download into your project.

You import Extensions like modules, so they are specific to each project. You will have to download your favorites when starting a new app. This also makes it easier for team members to use the same extensions when working on the same project. Any Extensions will be included when new team members download the app from the repository or merge changes into their existing copy of the project"