In the Post Content window, you can copy or enter text. To customize, highlight the text and a wysiwyg editor appears allowing you to change things like font size, bold, bullets and numbers.
You can also add pictures and embed video. When you click on Enter to start a new line, you will see the Plus sign appear. Click on the Plus sign and you will see the ability to upload an image, embed a video or add a break line.
Mobile works slightly different. In mobile, the editor stays on continuously.