Also, Linux Mint forums, Ubuntu forums and other close distros' forums - for questions that are likely to have been answered before. I prefer reading the thread myself rather than getting a summary, for it's a part of the fun - knowing there are lively communities and watching users suggest different approaches, good or bad, arguing, mentioning other topics that can come up later. It's not a systemic knowledge, rather case-by-case one, but it can be a path to some wider understanding of the OS.
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