Isn't that downvoting should follow a comment?
No, people are encouraged to explain their downvotes, but are not required to do so.
Or simply because you can downvote for free, you will do it?
If the question doesn't show sufficient prior research and/or at least some minimal effort to solve your problem on your own, of course I'll downvote it.
Now, specifically about your question:
- It's not a real question, it's a request for a list of resources. The "don't ask" section in the FAQ explains in some detail why we don't welcome such questions.
- No prior research or any other kind of effort is evident, the downvotes are justified.