Files are files so you can download them to your computer and upload them to the new provider. Unfortunately I am not aware of any direct provider-to-provider sync.

Application data depends on the app. For example, Notes [0] save your notes as Markdown files so you can move them (along with your files) wherever you want. However, News [1] don't and don't have export/import features at the moment either [2].

Nextcloud as a file storage solution and a non-collaborative office suite is great, but I cannot recommend its apps the same way. They are very convenient to install, but the quality varies a lot in my opinion so evaluate before you adopt.

[0] https://apps.nextcloud.com/apps/notes

[1] https://apps.nextcloud.com/apps/news

[2] https://github.com/nextcloud/news/issues/2503#issuecomment-2...