Question: if we copy anything from the Internet, and then just write that everything is taken from Google, is that correct?
At best, the user is not claiming the content as theirs, there are definitely better ways to go about it, but in the spirit of fairness; putting a source, regardless how sketchy or untidy would get one of the hook, as long as it is done before plagiarism is discovered.
And the second question: Everything on the forum is archived, why the user remains unpunished in case of obvious plagiarism and further editing? I mean after it became known to the public. Does it change something?
No it doesn't. If it did no one would ever be banned for plagiarism, they simply connect to a notifier like TryNinja's telegram bot and check am their quotes and mentions for accusation of plagiarism, then quickly add a source link before mods take action.
Adding a source after you make a post, but before you're caught out shows you did not intend to somewhat, but after it's discovered, you're just trying to cover your tracks.