These sources provide a comprehensive theoretical framework for various aspects of software development, specifically focusing on Web Application Architecture, Modern Web Development (with an emphasis on Blazor), and Mobile Development Architecture (with Xamarin). Each document begins by outlining the foundational principles and philosophy behind its respective area, tracing its historical context and evolution. They then offer a comparative analysis and trade-offs between different approaches within their domain, such as Blazor Server versus Blazor WebAssembly or Xamarin.iOS/Android versus Xamarin.Forms. A significant portion of each source is dedicated to a deep theoretical analysis, covering computer science foundations, software engineering principles, system architecture and performance theory, and domain-specific theoretical frameworks. Finally, the documents discuss practical implications and applications, future directions and research areas, and conclude by synthesising how this theoretical understanding enables informed design decisions, prediction of system behaviour, and quality assurance.