Ok, besides all those questions mentioned above, i guess, the easiest start would be to just go into your SunSky actor, select that mentioned Directional Light, and reduce it´s intensity to … dunno… 10, istead the 75000.
This will not be a realistic sun anymore in terms of intensity, but otherwise it will work the same. The moment you have to deal with real world light intensities from real world light bulbs, it will give you false results, but as a first starting point, and to make some older tutorials out there useable again (which were written before the SunSky with it´s 75000 light existed), it should be ok.
I have no clue, if there is an up to date ArchViz tutorial out there, that actually uses all this new stuff. Most seem to be using older stuff, which nowadays can lead to weird results, cuz much have changed during the last few releases of UE (like that SunSky Actor and the new atmosphere system).