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Hello,

I have read through the tutorials of "Create responsive layouts using Flex Grid" and "An Introduction to Flex Grid".

How do these work fom the server? For instance if someone clicks a SE link to a site using a desktop, tablet, or a cellphone, how does the appropriate page version show up?

Using a flexgrid is no different than using a layout grid, layers, etc. It's just another 'container' you can use to create a site with. It gives you some flexibility in creating a block layouts. You can add breakpoints to adjust the content for different size layouts the same way you would with using a grid, layer, etc.

Yes, I think I get that part. So is it specific to different devices? Do you have to put some redirect code on the page to take the user to the proper page?
Google wants websites to be mobile friendly. I think that usually mobile friendly sites pretty much atomatically adjust. Is that correct?

When using flex grids (and optional breakpoints), all content is part of the same page.
The browser will automatically select the correct layout.
So, there is no need for redirect code.