1. Automation and Continuous Integration (CI/CD) Platforms:

While their primary function isn't the direct addition of files by a user through an interface, tools like GitHub Actions, GitLab CI/CD, and Bitbucket Pipelines can be configured to add or modify files in your repository in response to certain events (e.g., a form submitted on a website). This requires prior configuration and isn't a direct user interaction to manually add files through a dedicated web interface.
2. How it typically works:

Login and Authorization: You log in to the site using your GitHub, GitLab, or Bitbucket credentials. The site then requests your authorization to access your account and potentially specific repositories.
Browsing and Editing: Once authorized, you can generally navigate your repositories. The interface allows you to create new files, edit existing files, and upload files from your computer. Commit and Push: When you save your changes or add new files, the site typically creates a new commit with your changes. You'll often have the option to write a commit message. The changes are then pushed to your remote repository on GitHub, GitLab, or Bitbucket.
3. Important points to consider:

Permissions: Be careful about the permissions you grant to these sites. Make sure you understand what level of access they are requesting for your account.
Security: Use trusted sites and check their privacy and security policies.
Workflow: These tools can simplify adding files in some cases, but for complex changes or large projects, using local Git tools is often the most powerful and flexible method.
In summary: Yes, several types of online sites can connect to your Git accounts and allow you to add files to your repositories after authorization, particularly online code editors and Git-based CMSs.