When creating properties and page types through admin mode in EPiServer, it's quite easy to understand how the database is written to in order to update the configuration of the website. But with page type builder, however, if it's as easy as writing...
[PageTypeProperty]
public virtual string MainBody { get; set; }
...to create a new property - at what point is this stored in the datbase? Runtime? Compile time?
Page Type Builder will create the properties at runtime.