How does one Perform a Crossfade between Scenes?

If you want a bunch of dissolves or crossfades between scenes you should be using an NLE like Premiere Pro. After Effects is not a video editing app. It is not suited to editing scenes together. It is designed to create visual effects shots, composites, motion graphics, and animations that you cannot create in an NLE. More than 90% of my comps are just one shot and that shot is usually less than seven seconds. Most of the movies I edit have hundreds of shots and I cut them in Premiere Pro or Davinci, or AVID or Final Cut Pro. I routinely create single-shot compositions that have dozens of layers. Sometimes they have more than 100 layers, but the only time I use more than one shot in a composition is when I have to do a transition between shots that cannot be created in an NLE.

For a simple fade between shots just animate opacity. If you want it to look a little different change the blend mode of the top layer to Alpha Add. 

There are other transitions in the Animation Presets that save time transitioning between layers. It all depends on what you are trying to do.