Salesforce has become one of the most popular cloud-based Customer Relationship Management (CRM) platforms in the world. It has been adopted by organisations of all sizes and industries globally. With its robust and scalable environment, Salesforce enables developers to build custom applications that meet their business needs. This guide is designed to provide a comprehensive resource for building scalable Salesforce applications using Apex data types, design patterns, and SOLID principles. It offers developers the opportunity to learn how to design and develop applications that are flexible, maintainable, and scalable. The guide covers a variety of topics, including the basics of Apex data types, object-oriented design principles, design patterns, and SOLID principles. It also provides practical examples and best practices that developers can apply to their applications. Whether you are an experienced Salesforce developer or just starting, this guide is intended to help you build better applications that are scalable and easy to maintain. We hope that this guide will serve as a valuable resource for developers and help them build better Salesforce applications.