To ensure the Azure Pipeline correctly caches and restores NuGet packages, the configuration must match standard caching patterns for dependencies:

• Target 1 (Key): The primary key identifies the cache. It is composed of the string "nuget", the operating system, and the unique hash of the lock file. You must drag $(Build.SourcesDirectory)/**/packages.lock.json here so the cache invalidates/updates whenever your dependencies change.
• Target 2 (restoreKeys): This field provides fallback keys if the exact primary key isn't found (e.g., if the lock file changed). The standard pattern is to fall back to the OS level. The line nuget | is already present, so you drag $(Agent.OS) here to complete the prefix nuget | $(Agent.OS). This allows the pipeline to restore the most recent cache for that OS, even if the lock file hash doesn't match perfectly, speeding up the subsequent restore.
• Target 3 (path): This defines where the files are stored. The variable NUGET_PACKAGES is defined earlier in the YAML as $(Pipeline.Workspace)/.nuget/packages. Therefore, the path to cache is $(NUGET_PACKAGES).

Microsoft Learn - Caching NuGet packages

Relevance: The official documentation explicitly demonstrates this 3-part pattern: using the lock file in the key, using the $(Agent.OS) in the restoreKeys, and using the $(NUGET_PACKAGES) variable for the path.

Source: Cache NuGet packages - Azure Pipelines https://learn.microsoft.com/en-us/azure/devops/pipelines/artifacts/caching-nuget