Filter geometry based on map extent using OpenLayers

I need to summarize an attribute of a geojson based on the map extent. I want to sum values for all features intersecting bbox. I think that I must use the Bbox filter but isn’t clear for me how I can use it.

I’ve a simple GeoJSON with four geometry named test_data.geojson:

{
"type": "FeatureCollection",
"name": "test_data",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
"features": [
{ "type": "Feature", "properties": { "id": 1, "name": "focusarea-id1", "pop": 30 }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ 10.224194550042373, 41.358221559689788 ], [ 10.224194550042373, 42.154498393805234 ], [ 11.816748218273265, 42.154498393805234 ], [ 11.816748218273265, 41.358221559689788 ], [ 10.224194550042373, 41.358221559689788 ] ] ] ] } },
{ "type": "Feature", "properties": { "id": 2, "name": "focusarea-id2", "pop": 20 }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ 11.960078048414047, 40.779593726899229 ], [ 11.960078048414047, 41.87314724575112 ], [ 13.297823129727995, 41.87314724575112 ], [ 13.297823129727995, 40.779593726899229 ], [ 11.960078048414047, 40.779593726899229 ] ] ] ] } },
{ "type": "Feature", "properties": { "id": 3, "name": "focusarea-id1", "pop": 10 }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ 11.519471533536832, 42.425232517404488 ], [ 11.519471533536832, 43.582488182985607 ], [ 12.8890676882154, 43.582488182985607 ], [ 12.8890676882154, 42.425232517404488 ], [ 11.519471533536832, 42.425232517404488 ] ] ] ] } },
{ "type": "Feature", "properties": { "id": 4, "name": "focusarea-id3", "pop": 40 }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ 13.043014542811051, 42.255360126126526 ], [ 13.043014542811051, 43.465700913981998 ], [ 13.871142450291117, 43.465700913981998 ], [ 13.871142450291117, 42.255360126126526 ], [ 13.043014542811051, 42.255360126126526 ] ] ] ] } }
]
}

I’m be able to put on the map my four rectangles using this code:

var sourceDataUrl = '{% static 'gis-data/vettori/test_data.geojson' %}';
var polygonsSource = new ol.source.Vector({
    url: sourceDataUrl,
    format: new ol.format.GeoJSON(),
});
var vectors = new ol.layer.Vector({
  source: polygonsSource,
});
map.addLayer(vectors);

var fetchFn = fetch(sourceDataUrl)
  .then(function(resp) {
    return resp.json();
  });

/// Summarize pop
fetchFn.then(function(json) {
    var features = new ol.format.GeoJSON().readFeatures(json, {
      dataProjection: 'EPSG:4326',
      featureProjection: map.getView().getProjection(),
    });

    var polygonPop = 0;
    for (var i=0; i<features.length; i++) {
      polygonPop += features[i].values_.pop;
    }
    console.log(polygonPop);

});

/// Compute the bounding box based on map extent
map.getView().on('change', function() {
    var extent3857 = map.getView().calculateExtent(map.getSize())
    var extent4326 = ol.proj.transformExtent(extent3857, 'EPSG:3857', 'EPSG:4326');
    console.log('Boundary Box 4326: '+extent4326);

});

I can compute the bounding box based on map extent but I don’t understand how I can link this feature with polygonSurface.

EDIT 1 with the indications of @Mike

map.getView().on('change', function() {

    var extent = ol.proj.transformExtent(
      map.getView().calculateExtent(map.getSize()),
      'EPSG:3857',
      'EPSG:4326',
    );

    var polygonIntersection = [];

    polygonsSource.forEachFeatureIntersectingExtent(extent, function(feature) {
      console.log(feature);
      polygonIntersection.push(feature);
    });

    console.log(polygonIntersection); // response -> Array []

});

Using the method forEachFeatureIntersectingExtent I’ve an empty array.

map.on('moveend', function() {
  var extent = map.getView().calculateExtent(map.getSize());
  var polygonIntersection = [];

  polygonsSource.forEachFeatureIntersectingExtent(extent, function(feature) {
    polygonIntersection.push(feature);
  });
  
  console.log(polygonIntersection);
});