Music Sync Library not working properly

[phl92]

How is Music Sync Library working? I mean where is Music stored, it seems not to be in iCloud Storage in my Case.

I had Sync Library deactivated until today, but I just downloaded some audio files on my MBA and imported them in the Music App on the Macbook. Because I want them to play also on my iPhone I activated the Sync Library option in the settings of my MBA.
Well, the audio files I want appear on my iPhone but are still not playable “Song not synced between your devices”
I guess this is because the Syncing process is not finished yet. I have around 20Gb music in total.
So my question is, will this syncing store music in between the Music App or also in my iCloud storage?

 

[phl92]

So today morning, from my 21 Audio files 12 of them are synced and work also on my iPhone. However 9 of them my Music says "The item was not added to your Music Cloud Library because an error occured".
Also on my MBA in the Music App I have to choose for each file now the location again. Why is this happening? Why Apple Cloud services work so faultily?
All files are on my MBA in a folder and were imported in the Music App. Why he is "losing" the files while syncing?

edit: Now I relocated all files in the Music APP on my Macbook, I saw that the App was for some minutes updating its Music Cloud Library but on my iPhone still nothing updated. Half of the audio have a small red cloud with an exclamation mark inside. When I click on it, it says only that I have to activate Sync Libarby on my Mac, which is already activated of course.