Although it is not possible to select multiple features simultaneously in an ArcGIS Online web map (as described in FAQ: Is it possible to select multiple features graphically in an ArcGIS Online web map?), it is possible to do so using the Select widget in Web AppBuilder for ArcGIS.
The images below show the distribution of wastewater treatment plants in South Island, New Zealand in point layers, and the location of active fault zones in South Island provided by GNS Science. Areas near or along the fault zones are prone to earthquakes, therefore there is a need to upgrade and construct the facilities in these areas with new building and structural codes to withstand earthquakes.
This article describes how to select features in Web AppBuilder for ArcGIS, and save them on the My Content tab in ArcGIS Online as a feature layer to use them in consequent web maps and web apps. In this example, wastewater treatment plants located near fault zones are selected simultaneously and these features are saved on the My Content tab.
Note: To select multiple features, multiple polygons (or other selecting methods such as rectangles and circles) can be drawn at a time. To do so, press the Shift key while selecting the features.
The following image is the result, which displays the feature layer with 19 wastewater treatment plants located near or along South Island fault zones that must be upgraded and constructed to withstand earthquakes.