exportKml

Exports an EntityCollection as a KML document. Only Point, Billboard, Model, Path, Polygon, Polyline geometries will be exported. Note that there is not a 1 to 1 mapping of Entity properties to KML Feature properties. For example, entity properties that are time dynamic but cannot be dynamic in KML are exported with their values at options.time or the beginning of the EntityCollection's time interval if not specified. For time-dynamic properties that are supported in KML, we use the samples if it is a SampledProperty otherwise we sample the value using the options.sampleDuration. Point, Billboard, Model and Path geometries with time-dynamic positions will be exported as gx:Track Features. Not all Materials are representable in KML, so for more advanced Materials just the primary color is used. Canvas objects are exported as PNG images.
Example:
Cesium.exportKml({
     entities: entityCollection
 })
  .then(function(result) {
    // The XML string is in result.kml

    const externalFiles = result.externalFiles
    for(const file in externalFiles) {
      // file is the name of the file used in the KML document as the href
      // externalFiles[file] is a blob with the contents of the file
    }
  });
Demo: