Select text to get a small toolbar: bold, italic, underline, strike, code, highlight, link and comment.
Or use the toolbar above the note: text styles (Normal text, Heading 1–3), text color, lists, checklist and link. Narrow screens put the rest under More.
Markdown shortcuts work as you type: #, ##, ### and a space for headings, - for a list, [] for a checklist, > for a quote, three backticks for code. Turn them off in Settings → Editor.