My group and I at the University of Gent are developing a project; we made a WebApp Online, we added some layers, but now we want them to appear just when selected. How can we do this? Also, when doing the presentation, we'd like the WebApp to appear with less zoom, I mean, show both the USA & Europe coast + UK + Iceland for example, instead of showing just the Belgian coast. I installed ArcGIS Desktop, but still can't figure it out. Here is the link of the webapp: http://ughent.maps.arcgis.com/apps/View/index.html?appid=3e9a6f9a2f4a42f999fc8c903076fae6
You can set the default map extent to whatever you desire in the Web Map builder (so when the map opens the extent will be zoomed out enough to view both continents).
In regards to only having selected layers to be shown, I am not exactly sure what you mean. Each layer can have a visibility range so that it is only turned "ON" when you are zoomed in or out at a specific range that you set. You can set this range the layer properties for the web map.
To build on @Joe answer, the two main entities you are working with are the WebMap and the WebApp. The map is where you configure the layers you want (scale range, default visibility, pop-up configuration, transparency, ect.) and the app is where you can set the initial extent, add in tools, and change the layout/design. The WebMap is the where you set up the data that will participate in the WebApp. Any tools or functions you wish to set up in the WebApp will reference the data you configured in the WebMap.
As for your specific questions, I am also a bit confused about the "show only when selected". To set the visibility range, in the WebMap click on the 3-dots under the layer you want to alter and choose 'Set Visibility Range'. Experiment with the slider until you find the setting you are looking for.
There is also a widget you can add into the WebApp called Query. This can be set up to show/highlight all (or a queried selection) of the features in a layer. For example, it can be set up to highlight all park polygons, even if the layer is set not to draw at the current scale range. Is this what you are looking for?
For setting the initial extent of the WebApp when it opens, while you are configuring the WebApp, under the Map tab, there is a section that reads 'Set initial Extent'. Of the two option, the one you are looking for it 'Use Current map View'. In the map viewer window, zoom and pan to where ever you want the extent to be, and then click that button.
Hope this is helpful.