These are the five methods currently believed to work for syncing between devices if you’ll be including an iPhone and/or iPad:
• iCloud (NOTE: iCloud on Windows is known to cause file duplication issues as well as data corruption and loss, and the Obsidian devs and community strongly recommend that you avoid this combination.)
• Official Obsidian Sync
• Self-hosted LiveSync plugin
• Remotely Save plugin and various cloud services, including Amazon S3 or S3-compatible, Dropbox, OneDrive for personal, and Webdav
Yes, iCloud is only viable for syncing Obsidian between devices running Apple operating systems. It will corrupt or destroy your data on Windows and won’t work at all on Android or Linux.