Yes, i understand your frustration and yes it is not an easy task.
Surely there are also other options, i just showed you one possible way to work with UPLOADING FILES to your Wix-Manager.
Did you also read the SECOND post which is connected to my first one?
Here you will find the right APIs you can use for your purpose…
https://www.wix.com/velo/reference/wix-media-backend/mediamanager-obj
If they are more clear for you, maybe they will help you better to find your solution.
For sure such CODINGS are nothing for beginners, even not for advanced CODERS, i would say. Such CODES are for the programmers who are already experienced enough to solve every-coding-problem on their own, even if it will take more than a week of time to solve it.
You ask if there are TUTORIALS for such CODINGS ???
I don’t think so. It would take weeks to create a good tutorial for this topic.
If there is someone out there who wants to work sevral weeks for free, feel free to do it.