External keyboard required. Word's keyboard shortcuts on iPhone only activate with a physical keyboard connected.
Text formatting shortcuts. Bold, italic, underline, and basic paragraph formatting shortcuts are the most consistently useful on iPhone. These are the same key combinations as on desktop (Cmd+B, Cmd+I, Cmd+U on a connected keyboard following iOS conventions).
Document navigation. Navigation shortcuts for moving by word, jumping to the beginning or end of the document, and scrolling within long documents are available with an external keyboard.
Discovering shortcuts. Holding the Command key on a connected Bluetooth keyboard in Word for iOS shows the available shortcuts overlay, a quick reference for what's supported in the current context.
Tracked changes. Word on iOS supports accepting and rejecting tracked changes. The shortcut set for this is more limited than on the desktop app.