KML and KMZ

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KML is the XML format of Google Earth and Google My Maps, and KMZ is a zip file that holds a KML document and its images. Kml reads and writes KML, and Kmz reads and writes KMZ:

GeoFeatureCollection placemarks;
using (var stream = File.OpenRead("trip.kml"))
{
    placemarks = Kml.Read(stream);
}

// The placemarks of every folder, in order; the "folder" property gives the folder of each one
Assert.Equal("Trip", placemarks.Name);
var monuments = placemarks.Where(placemark => (string?)placemark.Properties.GetValueOrDefault("folder") == "Monuments").ToList();
var eiffelTower = Assert.Single(monuments);
Assert.Equal("Eiffel Tower", eiffelTower.Properties["name"]);
Assert.Equal("<b>330</b> m", eiffelTower.Properties["description"]);
Assert.Equal("1889", eiffelTower.Properties["year"]); // KML has no types: ExtendedData values are strings

using (var stream = File.Create("monuments.kml"))
{
    Kml.Write(new GeoFeatureCollection(monuments) { Name = "Monuments" }, stream, new KmlWriterOptions { Indented = true });
}

using (var stream = File.Create("trip.kmz"))
{
    Kmz.Write(placemarks, stream);
}

using (var stream = File.OpenRead("trip.kmz"))
{
    var copy = Kmz.Read(stream);
    Assert.HasCount(2, copy);
}

What the reader keeps

Every Placemark, in the folders at any depth, in the order of the document. The name of the document becomes the Name of the collection, and the id of a placemark its Id.
The properties: name, description (its HTML is kept as text), the items of ExtendedData, and folder, the names of the folders that contain the placemark, separated by /. Google My Maps puts each layer in a folder.
KML has no types: the values are strings, except the SimpleData items that a Schema declares as numbers (double) or booleans (bool).
The geometries: points, lines, polygons, MultiGeometry, and the gx:Track and gx:MultiTrack tracks of Google Earth, as lines. A LinearRing alone is a line, as Google Earth draws it. The altitudes are kept as they are written, whatever their altitudeMode.
The when elements of a gx:Track are the Time of its positions, in order. A when that has no time of day, such as a date alone, gives no time.
The styles, the time spans and time stamps of the placemarks, the 3D models, the overlays and the network links are ignored: nothing is downloaded.
A KMZ file is read from its doc.kml entry, or else its first .kml entry. The other files are ignored.

What the writer writes

A KML 2.2 document with a Placemark per feature, also for the features without a geometry.
The name and description of a placemark come from the string properties named by NameProperty and DescriptionProperty (name and description by default). The other properties are ExtendedData items, as text.
The identifier is written when it is a valid XML identifier: a number, such as the id of a GeoJSON feature, is not.
A geometry with altitudes gets the absolute altitude mode; otherwise, Google Earth draws it on the ground.
A line whose positions all have a time is a gx:Track, and a multi-line of such lines a gx:MultiTrack. The times of the other positions, and the measures, are not written.
The folders are not written back: the folder property is an ExtendedData item like the others.