Short answer: The author should retain ownership and control of the manuscript and research. Editing support should improve presentation without transferring authorship or ownership of the underlying academic work.
Before sending unpublished work, review the provider’s privacy and confidentiality terms. Ask how files are stored, who can access them, and how long they are retained when that matters to your project.
Editors should not add themselves as authors merely for language editing. Authorship should follow the journal’s and institution’s applicable authorship criteria.
Keep your original file and review all tracked changes before submission. The author remains responsible for the final manuscript.