As a candidate for the Microsoft DP-420 exam, you need solid knowledge and experience developing apps for Azure and working with Azure Cosmos DB database technologies. You should also be proficient at developing applications by using the Core (SQL) API and software development kits (SDKs), writing efficient queries, and creating appropriate index policies. You need experience provisioning and managing resources in Azure and creating server-side objects with JavaScript.
Plus, you need to be able to interpret JSON, read C# or Java code, and use PowerShell.