Check out example codes for "c# cosmos db add items into container". It will help you in understanding the concepts better.

Code Example 1

using Microsoft.Azure.Cosmos

public class Test
{
	[JsonProperty(PropertyName = "CategoryId")]
	public int Id { get; set; }
}

public async Task AddItemsAsync(CosmosClient cosmosClient, Database database, Container container)
{

	Test test = new Test();
	test.Id = 0;

	ItemResponse<Test> testResponse = await container.CreateItemAsync<Test>(test, new PartitionKey("<Your partition key>"));

	Console.WriteLine("Created item in database with id: {0} Operation consumed {1} RUs.\n"
		, testResponse.Resource.Id, testResponse.RequestCharge);
}

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