how do I achieve lighting as it’s shown in the youtube example ?
I have created a room with massive lakefront window but I’d like to use as much “real” light as possible to light my room…
is this the sun angled to come in through the window… along with Global Illumination ?
if I follow this tutorial,. might that be the best place to start ?
what is causing the light source outside the example above ?
The video you linked above is from a user who posts here on the forums.
Try asking some questions in their forum and perhaps they will give you the setup they specifically use to get their lighting.
Rafareis: Modern Kitchen: https://forums.unrealengine.com/showthread.php?35959-Modern-Kitchen
Koola: ArchViz/Lighting: https://forums.unrealengine.com/showthread.php?28163-ArchViz-Lighting
If you look at Epics Realistic Rendering that is available on the Marketplace you will see that we’re using spot lights for our lighting coming through the windows.
I’m not sure how the two above are specifically setup but they may be willing to share some tips. 
Tim