Now easy to migrate your data from the following services to your Nextcloud account.
Note: in all cases, the administrator has to install the corresponding app first!

• OneDrive. To be able to migrate data, the Nextcloud instance administrator also has to setup OAuth credentials with OneDrive for their server.
• Dropbox. Connecting it to your account should just work.
• Google. To be able to migrate data, the Nextcloud instance administrator also has to setup OAuth credentials with Google for their server.

Also see Nextcloud's press release, and Forbes article.

https://nextcloud.com/blog/easy-migration-to-nextcloud-from-insecure-and-privacy-unfriendly-platforms-now-available/