Formatting text in Moodle books in courses built on OpenLearn Create is done with the TinyMCE editor.
The editor includes icons for various formatting functions. You can click on the Reveal or hide additional toolbar items icon (three dots at the end of the first row) to reveal a second and third row of functions.

These functions, as well as many others, are also available in the options at the top of the editing window: Edit, View, Insert, Format, Tools, Table and Help.
The size of the editing screen can be lengthened by dragging the Resize function at the bottom right of the editing window.
You can also expand the editing window to match your screen's dimensions, which you may find useful over the following pages when you are adding images or videos. To do this, go to View and then Fullscreen. Repeat this to return to the default size editing window.