How is feedback process done after submitting an article to arXiv?
There is no official protocol or formal procedure. Here is how I handle it. 

I skim the arxiv submission in my fields of interest every day (headlines and abstracts). Every few days I find some things that I read in greater detail and if it happens that I find something that I'd like to discuss with the authors, I write a joint email to all of them. Could be a question for clarification, a hint for further reading or something like "look here, I've done something related". Once in while I take the time to write a blog post for some article and then it sometimes (rarely) happens that one of the authors comments on my blog.