Use the Note Pad in GarageBand for iPhone
You can create, view, and edit notes for a song with the Note Pad. Each song can contain one set of notes.
Tap the Settings button in the control bar, then tap Note Pad.
To start typing, tap inside the text area.
On iPhone SE, iPhone 8, iPhone 7, or iPhone 6s, tap the Settings button in the control bar, tap Song Settings, then tap Note Pad.