Azure Documentation is the official and one of the best resources to know about any Azure topic in detail. If you are preparing for AZ-900 certification then these Docs references will surely help you to get deeper understanding of each topic from exam curriculum.
Here is the list of topics with their references:
Identify the benefits and considerations of using cloud services

• Identify the benefits of cloud computing, such as High Availability, Scalability, Elasticity,Agility, and Disaster Recovery
• Identify the differences between Capital Expenditure (CapEx) and OperationalExpenditure (OpEx)
• Describe the consumption-based model

Describe the differences between categories of cloud services

• Describe the shared responsibility model
• Describe Infrastructure-as-a-Service (IaaS)
• Describe Platform-as-a-Service (PaaS)
• Describe serverless computing
• Describe Software-as-a-Service (SaaS)
• Identify a service type based on a use case

Describe the differences between types of cloud computing

• Define cloud computing
• Describe Public cloud
• Describe Private cloud
• Describe Hybrid cloud
• Compare and contrast the three types of cloud computing

Describe the core Azure architectural components

• Describe the benefits and usage of Regions and Region Pairs
• Describe the benefits and usage of Availability Zones
• Describe the benefits and usage of Resource Groups
• Describe the benefits and usage of Subscriptions
• Describe the benefits and usage of Management Groups
• Describe the benefits and usage of Azure Resource Manager
• Explain Azure resources

Describe core resources available in Azure

• Describe the benefits and usage of Virtual Machines, Azure App Services, AzureContainer Instances (ACI), Azure Kubernetes Service (AKS), and Windows Virtual Desktop
• Describe the benefits and usage of Virtual Networks, VPN Gateway, Virtual Network peering, and ExpressRoute
• Describe the benefits and usage of Container (Blob) Storage, Disk Storage, File Storage, and storage tiers
• Describe the benefits and usage of Cosmos DB, Azure SQL Database, Azure Database forMySQL, Azure Database for PostgreSQL, and SQL Managed Instance
• Describe the benefits and usage of Azure Marketplace

Describe core solutions available in Azure

• Describe the benefits and usage of Internet of Things (IoT) Hub, IoT Central, and Azure Sphere
• Describe the benefits and usage of Azure Synapse Analytics, HDInsight, and Azure Databricks
• Describe the benefits and usage of Azure Machine Learning, Cognitive Services and Azure Bot Service
• Describe the benefits and usage of serverless computing solutions that include Azure Functions and Logic Apps
• Describe the benefits and usage of Azure DevOps, GitHub, GitHub Actions, and Azure DevTest Labs

Describe Azure management tools

• Describe the functionality and usage of the Azure Portal, Azure PowerShell, Azure CLI, Cloud Shell, and Azure Mobile App
• Describe the functionality and usage of Azure Advisor
• Describe the functionality and usage of Azure Resource Manager (ARM) templates
• Describe the functionality and usage of Azure Monitor
• Describe the functionality and usage of Azure Service Health

Describe Azure security features

• Describe basic features of Azure Security Center, including policy compliance, security alerts, secure score, and resource hygiene
• Describe the functionality and usage of Key Vault
• Describe the functionality and usage of Azure Sentinel
• Describe the functionality and usage of Azure Dedicated Hosts

Describe Azure network security

• Describe the concept of defense in depth
• Describe the functionality and usage of Network Security Groups (NSG)
• Describe the functionality and usage of Azure Firewall
• Describe the functionality and usage of Azure DDoS protection

Describe core Azure identity services

• Explain the difference between authentication and authorization
• Define Azure Active Directory
• Describe the functionality and usage of Azure Active Directory
• Describe the functionality and usage of Conditional Access, Multi-Factor Authentication(MFA), and Single Sign-On (SSO)

Describe Azure governance features

• Describe the functionality and usage of Role-Based Access Control (RBAC)
• Describe the functionality and usage of resource locks
• Describe the functionality and usage of tags
• Describe the functionality and usage of Azure Policy
• Describe the functionality and usage of Azure Blueprints
• Describe the Cloud Adoption Framework for Azure

Describe privacy and compliance resources

• Describe the Microsoft core tenets of Security, Privacy, and Compliance
• Describe the purpose of the Microsoft Privacy Statement, Online Services Terms (OST) and Data Protection Amendment (DPA)
• Describe the purpose of the Trust Center
• Describe the purpose of the Azure compliance documentation
• Describe the purpose of Azure Sovereign Regions (Azure Government cloud services and Azure China cloud services)

Describe methods for planning and managing costs

• Identify factors that can affect costs (resource types, services, locations, ingress and egress traffic)
• Identify factors that can reduce costs (reserved instances, reserved capacity, hybrid use benefit, spot pricing)
• Describe the functionality and usage of the Pricing calculator and the Total Cost of Ownership (TCO) calculator
• Describe the functionality and usage of Azure Cost Management

Describe Azure Service Level Agreements (SLAs) and service lifecycles

• Describe the purpose of an Azure Service Level Agreement (SLA)
• Identify actions that can impact an SLA (i.e. Availability Zones)
• Describe the service lifecycle in Azure (Public Preview and General Availability)