Plain text – by definition – cannot display formatted text or multimedia elements. When you use any of the formatting buttons or try to insert an image into a plain text document, Notebooks adds Markdown code, which may look strange at first. When you switch to the document’s formatted version (on iOS, tap the action button an choose Process > Format Document…; on Mac or PC click Formatted View) you see the formatted representation of that document including the correct formatting and multimedia elements. From here you can convert the plain text to a formatted document and continue working on that, but you can just as well stick with the Markdown version of the document.
Strange Symbols When Inserting Photos in Plain Text