How I can configure to set to sync folders in Users (like My Documents, My Downloads, etc) like in One Drive ?
You can set up more than one sync and point them to different folders. For example uncheck Nextcloud’s Documents folder on the original sync, then set up a second sync to go from the computer’s My Documents to Nextcloud’s Documents folder.
https://docs.nextcloud.com/desktop/latest/navigating.html#configuring-nextcloud-account-settings
for my understandig you don’t need to create multiple sync sets, just change path on the explorer folder properties (there is a Path tab) for the folders you want to change and when you hit Apply you can locate to a folder in your nextcloud folder.
Unfortunately some applications don’t like the way how nextcloud changes/locks files. At least with “Microsoft Journal” from the Microsoft Store for hand writing notes you get corrupted documents, which works with OneDrive. I don’t know if there are other pitfalls.