Infrastructure-as-Code (IaC) has emerged as a foundational paradigm for automating the provisioning, configuration, and management of cloud and network infrastructures at scale. By representing infrastructure through machine-readable code, IaC enables repeatability, consistency, scalability, and seamless integration with DevOps and Site Reliability Engineering (SRE) practices. As enterprise infrastructures increasingly span hybrid, multi-cloud, and distributed environments, selecting and deploying appropriate IaC tools has become both critical and challenging. This paper presents a comprehensive survey of Infrastructure-as-Code tools, architectures, and deployment strategies for large-scale cloud and network automation. It systematically examines open-source and commercial IaC solutions, including Terraform, Ansible, Pulumi, Puppet, Chef, CloudFormation, and cloud-native platforms, comparing their design principles, scalability, and suitability for cloud and network automation. The study analyzes declarative and imperative models, centralized and distributed automation architectures, and integration with CI/CD pipelines. Key applications such as cloud provisioning, network orchestration, continuous deployment, and security and compliance automation are discussed. Furthermore, the paper identifies major challenges related to scalability, state management, security, standardization, and interoperability, and outlines emerging research directions including AI-driven IaC, edge and IoT automation, serverless infrastructure management, and standardized reference architectures. By consolidating tools, architectural models, and best practices, this survey provides a structured reference for researchers and practitioners designing scalable, reliable, and secure automation frameworks for modern cloud and network infrastructures.