What is the difference between “sync” and “download” in the Android client?
Sync is “just” synchronizing the server information to the client (new files/folders information, just the meta data) while download is actually downloading the file to your device.
Thanks. Though in my case, sync deleted my local copy and downloaded all files in the folder again - several GB of files… Am I doing something wrong maybe?
My mistake “Sync” on a folder simply downloads the server’s folder to the client 
Sorry for the confusion.
No problem. I was on WiFi and nothing was lost in the end. Is this a bug that it removed all existing local content? I think that a warning should be in place.
What was the existing local content you had in that folder?
No, it was content previously downloaded from the server with the NC app with “Sync” or “Download”. The files had not been changed at all.