Example:

{
  $geometry: {
    type: "GeometryCollection",
    geometries: [
      {
        type: "MultiPoint",
        coordinates: [
          [ -73.9580, 40.8003 ],
          [ -73.9498, 40.7968 ],
          [ -73.9737, 40.7648 ],
          [ -73.9814, 40.7681 ]
        ]
      },
      {
        type: "MultiLineString",
        coordinates: [
          [ [ -73.96943, 40.78519 ], [ -73.96082, 40.78095 ] ],
          [ [ -73.96415, 40.79229 ], [ -73.95544, 40.78854 ] ],
          [ [ -73.97162, 40.78205 ], [ -73.96374, 40.77715 ] ],
          [ [ -73.97880, 40.77247 ], [ -73.97036, 40.76811 ] ]
        ]
      }
    ]
  }
}

https://www.mongodb.com/docs/manual/reference/geojson/#geometrycollection