My curated list
There are plenty of resources out there to help you study for the AZ-900 exam. The exam is for candidates who are just beginning to work with cloud-based solutions and services.
It's an opportunity to gather knowledge of cloud concepts, services, workloads, security, privacy, pricing and support. Here comes my list:
Elastic https://azure.microsoft.com/en-us/overview/what-is-elastic-computing/
Consumption and fixed cost models https://docs.microsoft.com/en-us/azure/architecture/framework/cost/design-price
Shared responsibility https://docs.microsoft.com/en-us/azure/security/fundamentals/shared-responsibility
Cloud Computing https://docs.microsoft.com/en-us/learn/modules/fundamental-azure-concepts/types-of-cloud-computing
IAAS https://azure.microsoft.com/en-us/overview/what-is-iaas/
PAAS https://azure.microsoft.com/en-us/overview/what-is-paas/
SAAS https://azure.microsoft.com/en-us/overview/what-is-saas/
Cloud Models https://azure.microsoft.com/en-us/overview/what-are-private-public-hybrid-clouds/
Public cloud https://azure.microsoft.com/en-gb/overview/what-is-a-public-cloud/
Geographies https://azure.microsoft.com/en-us/global-infrastructure/geographies/#overview
Region and availability zones https://docs.microsoft.com/en-us/learn/modules/azure-architecture-fundamentals/regions-availability-zones
Compute https://azure.microsoft.com/en-us/product-categories/compute/
Virtual machines https://docs.microsoft.com/en-us/learn/modules/azure-compute-fundamentals/azure-virtual-machines
Serverless https://docs.microsoft.com/en-us/learn/modules/azure-compute-fundamentals/azure-functions
Previews https://azure.microsoft.com/en-us/support/legal/preview-supplemental-terms/
Pricing https://azure.microsoft.com/en-us/pricing/details/bandwidth/ https://www.microsoft.com/en-us/security/business/identity-access-management/azure-ad-pricing
https://docs.microsoft.com/en-us/learn/modules/plan-manage-azure-costs/
https://azure.microsoft.com/en-us/pricing/calculator/
https://azure.microsoft.com/en-us/pricing/tco/calculator/
Big Data https://docs.microsoft.com/en-us/azure/architecture/guide/architecture-styles/big-data
IoT https://docs.microsoft.com/en-us/azure/iot-fundamentals/iot-introduction
Cognitive Services https://azure.microsoft.com/en-us/services/cognitive-services/
Strategy https://docs.microsoft.com/en-us/learn/modules/build-cloud-governance-strategy-azure/
Resource Groups https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/overview#resource-groups
Azure Management Groups https://docs.microsoft.com/en-us/azure/governance/management-groups/overview
Policy https://docs.microsoft.com/en-us/azure/governance/policy/overview
Azure AD https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis
SSO https://docs.microsoft.com/en-us/azure/active-directory/manage-apps/what-is-single-sign-on
MFA https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-mfa-howitworks
Azure Advisor https://docs.microsoft.com/en-us/azure/advisor/
Azure Cloud Shell https://docs.microsoft.com/en-us/azure/cloud-shell/overview
Azure Cost Management https://docs.microsoft.com/en-us/learn/paths/az-900-describe-azure-cost-management-service-level-agreements/
Azure Key Vault https://docs.microsoft.com/en-us/azure/key-vault/
Azure Monitor https://docs.microsoft.com/en-us/azure/azure-monitor/overview
Azure Resource Manager https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/overview
Azure Service Health https://docs.microsoft.com/en-us/azure/service-health/overview
Azure Service Trust https://servicetrust.microsoft.com/
Network https://docs.microsoft.com/en-us/azure/security/fundamentals/network-overview
CDN https://docs.microsoft.com/en-us/azure/cdn/cdn-overview
Load Balancing https://docs.microsoft.com/en-us/azure/architecture/guide/technology-choices/load-balancing-overview
Defense in depth https://docs.microsoft.com/en-us/learn/modules/secure-network-connectivity-azure/2-what-is-defense-in-depth
Perimeter networks https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/ready/azure-best-practices/perimeter-networks
Network Security https://docs.microsoft.com/en-us/azure/cloud-adoption-framework/ready/considerations/networking-options
Network Security Groups https://docs.microsoft.com/en-us/azure/virtual-network/network-security-groups-overview
DDoS https://docs.microsoft.com/en-us/azure/ddos-protection/ddos-protection-overview