Custom Page

Hi I am wanting to make a custom theme page and understand the dynamics, I think, but what all the articles don’t mention is how to include the css. My understanding is to get a single.php file, copy it to child theme and then put the appropriate php code in the top, and then paste in the html but where is all the styling, is it included in the html. As a web designer, I am used to designing page in html and attaching a style sheet.

Apologies for my confusion but none of the articles or videos explain this bit.

Using the Twenty Eleven theme. Thanks