Start learning how the Mendix Metamodel works by reading Understanding the Mendix Metamodel.
Continue with Projects (Metamodel) to learn how the Mendix Metamodel handles projects in relation to modules and documents.
Next, move on to learning how a Domain Model (Metamodel) works with all its components (attributes, access rules, etc.).
The Pages (Metamodel) page explains how pages, layouts, and page content are structured.
The Microflows (Metamodel) page describes in detail how you can create and invoke microflows.