They put detailed codes into their post, usually after splitting the post in two and put the code/information in the bottom section so the code doesn't change any pages the bog shows up on as well.

Type in CSS in the search and you'll find blogs made to help you make your own and/or a few layout codes everyone is able to use and edit for themselves. Also some people take requests for more unique and custom blogs.

the layout code is the CSS code used for layouts, that's all. Just make a blog saying you're wanting a code for your bio and someone might post saying they'll do one for you. They'll also tell you how to insert it.