object/nightwatch/26 matches the site

public static HumanMadeObject Nightwatch_Improvements()
{
    var nightWatch = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/nightwatch/26")
        .WithLabel("The Night Watch")
        .WithClassifiedAs(Aat.Painting, Aat.TypeOfWork);

    nightWatch.ModifiedBy = [
        new Modification()
        {
            Label = "Conservation of Night Watch",
            ReferredToBy = [
                new LinguisticObject()
                    .WithClassifiedAs(Aat.Description, Aat.Statement)
                    .WithContent("Minor conservation work in response to earlier survey")
            ],
            Technique = [Aat.Term("300053027", "Cleaning")],
            TimeSpan = new LinkedArtTimeSpan
            {
                BeginOfTheBegin = new LinkedArtDate(2020, 1, 1),
                EndOfTheEnd = new LinkedArtDate(2021, 12, 31).LastSecondOfDay()
            },
            CarriedOutBy = [
                new Group()
                    .WithId("https://linked.art/example/group/rijksmuseum")
                    .WithLabel("Rijksmuseum")
            ],
            PartOf = [
                new Activity()
                    .WithId("https://linked.art/example/event/operation_nightwatch")
                    .WithLabel("Operation Night Watch")
            ]
        }
    ];

    return nightWatch;
}

• Metatype classification The second argument classifies the first, not the object: WithClassifiedAs(Aat.Description, Aat.Statement) says this is a Description, and Description is a kind of Statement. The Plain C# form shows the nesting.
• Terms outside the registry The vocabulary registry holds only the terms linked.art itself publishes, so a dataset's own concepts, places and people are written with Aat.Term, Tgn.Place and Ulan.Person, giving the id and the label.

var humanMadeObject = new HumanMadeObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/object/nightwatch/26",
    Label = "The Night Watch",
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300033618", "Painting")
        {
            ClassifiedAs = [Aat.TypeOfWork],
        },
    ],
    ModifiedBy =
    [
        new Modification
        {
            Label = "Conservation of Night Watch",
            CarriedOutBy =
            [
                new Group
                {
                    Id = "https://linked.art/example/group/rijksmuseum",
                    Label = "Rijksmuseum",
                },
            ],
            TimeSpan = new LinkedArtTimeSpan
            {
                BeginOfTheBegin = new LinkedArtDate(2020, 1, 1),
                EndOfTheEnd = new LinkedArtDate(2021, 12, 31, 23, 59, 59),
            },
            ReferredToBy =
            [
                new LinguisticObject
                {
                    Content = "Minor conservation work in response to earlier survey",
                    ClassifiedAs =
                    [
                        new Type("http://vocab.getty.edu/aat/300435416", "Description")
                        {
                            ClassifiedAs = [Aat.Statement],
                        },
                    ],
                },
            ],
            PartOf =
            [
                new Activity
                {
                    Id = "https://linked.art/example/event/operation_nightwatch",
                    Label = "Operation Night Watch",
                },
            ],
            Technique = [Aat.Term("300053027", "Cleaning")],
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.