I recently discovered that there are hundreds of Stack Exchange sites, and I joined the ones that I think I need. After posting my first question and/or answer in all sites, I realised that my post is always upvoted once it is given. Even if I get some downvotes after that upvote. So is that the website's work or it is community members?
No, this is not automatic. Your first post on each Stack Exchange site is reviewed by other users in the First Posts review queue and users are encouraged to vote (up or down) those posts. On average, first posts by new users are lower quality, so if you write a decent post, chances are that reviewers are positively surprised and upvote your post, even though it might not be useful for themselves.