The Available Data section of a screen's sections tells you which types of data are being sent to the current screen from other screens. If data is missing or conflicting, this section will tell you which data types are having issues and which screens might be causing the issue.
Magic Text is information pulled from records within your database. This can be anything from names, numbers, dates, or photo URLs. Think of Magic Text as a placeholder for information that will change based on a user's actions or information.
Magic Text can be accessed anywhere you see the sparkly T icon or where you see the sparkly X icon which is used in number fields and formulas. Upon clicking, it will open up menus for you to choose from your collections the pieces of data you want to display.
A modal is like a popup that serves as either an intermediate screen or a temporary screen. It's typically used to present options to the users about records, ask them to confirm something, or even edit records with a form of some kind.