So it involves all these huuuuge Zonai „paintings“ that are scattered all over the landscape - called geoglyphs.
You‘ll need to investigate them as each one will trigger a (quite incredible, and important) cutscene.
But to get going, you first need to find and speak to Impa (old Sheika lady who lived in Kakariko in BOTW). She‘s the one already investigating the geoglyphs, and she will set you off on your way.
This is where you will find her:
Head West and North from Lookout Landing to reach the New Serenne Stable in the Hyrule Ridge area, which is on the path towards another Skyview Tower. Nearby in the field you’ll see Impa standing by the large glyph.
Again, if you complete this side quest, not only will you get „the full picture“, but also a reward you won‘t want to miss