Microsoft turns on OneDrive backups by default without your knowledge or consent, sending your private files to MS servers, not only during setup but it can also happen during updates. Even when you try keep it disabled, they constantly pressure you into enabling it every time you login, and make it really easy to accidentally turn it on. Once your files are in OneDrive, MS can access them. Don't be surprised if it later turns out they're training their LLMs on people's private files.