Azure Developer CLI (azd) and Azure CLI (az) are both command-line tools, but they help you do different tasks.
azd focuses on the high level developer workflow. Apart from provisioning/managing Azure resources, azd helps to stitch cloud components, local development configuration, and pipeline automation together into a complete solution.
Azure CLI is a control plane tool for creating and administering Azure infrastructure such as virtual machines, virtual networks, and storage. The Azure CLI is designed around granular commands for specific administrative tasks.