I'm not going to try and answer all of your questions, but here's a little info:
Host skins are installed by the Host account and are available to be used by any portal within the DNN install. There could be hundreds or thousands of portals installed under that one code base. Site skins, on the other hand, are installed by the Host or Admin under the current portal and are available to be used by that portal only.
Keep in mind that if you are logged-in as Host, you can work on the current portal or the group of all portals under that program installation. Host has all the rights of Admin on every portal, plus the ability to make global settings that affect all portals. When logged-in as Admin, you can work on that portal only.
Now, in order to select and set the default skin to be used on the current portal instance, go to Admin --> Site Settings, scroll down to Appearance and choose which skin and container you'd like to use as a default on portal pages and Admin pages. Click the "update" link at the bottom of the page to save your changes.
Now, if you had only one page with "wide" content, you could go to that page and in the Control Panel up top, choose Settings --> scroll down to Advanced Settings, then choose a different skin and/or default container for that page only.
DotNetNuke is a very powerful web application, and reading the documentation is time well-spent. I wouldn't dream of cranking up AutoCAD and trying to create a drawing without doing some study up-front.
Good luck!