These methods may be preferable if you’re offloading data from a device, i.e., one that won’t be kept in sync going forward. You can upload individual files, or entire directory structures through a browser without having to install any special client software or browser add-ons.
You can upload files directly through your browser by clicking on the New button in the Files app. A File Open dialog will appear. You won’t be able to select folders, but you can select multiple files by Ctrl-clicking.
Another option is to simply drag and drop files and folders from your file manager into your browser.
For devices with local files that will be kept in sync with the cloud server going forward, it’s simpler to just set up the Nextcloud Desktop app and let it handle the initial upload/migration automatically: