Take a look at the website called "Planet minecraft", there are thousands of Minecraft resources you can browse and download
If you have ever heard of the syndicate project, he has a pretty good world. Just search the Minecraft project and go to one of the episodes that is a multiple of ten.
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Look up Minecraft Seed spotlights on YouTube.
On most websites, Minecraft maps are free. Some websites might charge you for downloads, so be sure to review all information given by the site, and if you're still unsure you can contact an administrator.
Minecraft videos are for entertainment, how to play minecraft and etc.
Not inherently, no. However if someone downloads the wrong file (one claiming to be a mod but is actually a virus), then some damage could be caused. Only download mods from reputable sites.
Yes. It doesn't matter if it's LAN I believe. You just have to download some plugins and put it in the plugins server if you're using vanilla minecraft.