Hello,

you would edit a page or a post in the Visual Editor, like below.
https://postimg.cc/9r8LrnkW

The Visual Editor icons allow you to format text, insert lists and links, and more.
Now, how can you access this code in WordPress? Through switching from the Visual Editor to the Text Editor (you are still working with the Classic Editor).
https://postimg.cc/jLPjjp2f

For CSS Go to Theme Setting -> Custom CSS.

Best Regards.