Exploring Umbraco 8 and playing around building a test site etc.

Building document types and content pages via the back office interface, and looking in the code itself I've noticed a files called models.generated.cs.

looking inside, it contains partial classes like: public partial class AndyTestPage : PublishedContentModel { }

and properties I've given it like :

[global::System.CodeDom.Compiler.GeneratedCodeAttribute("Umbraco.ModelsBuilder", "8.5.5")] [ImplementPropertyType("andyProperty1")] public string AndyProperty1 => this.Value

my question is, what exactly is the point of this file? what does is do and how are these properties and classes created in here via the backoffice cms.

second question is, how would I go about creating a document type, and properties from code? my noob assumption is to simply add the classes in the this file, but that doesnt seem right?

Thanks,

Andy