I got a bit confused because I do not see a straightforward way to do this.
For example, I have multiple folders like “D:\Documents”, “E:\Photos” on my Windows machine and I want to start syncing them with NextCloud. Effectively, I want to have up-to-date backups on the cloud.
Nextcloud is not a backup solution. It’s for syncing files (if you delete or change a file, this change is carried out on the server and all clients as well).
Click on the three dots and remove the default sync folder. Then the “Add folder”-button is active and you can define several sync points on your system.