First half: Nahida + Yoimiya
Second half: Yae + Childe + Layla
Yoimiya had always been put between a rock and a tight place. Her first banner was right before the electro archon Raiden Shogun's banner. And her rerun was right before Sumeru. Moreover on her rerun, Yoimiya was put alone with no new characters, while Kazuha, Klee, and Heizou were put together on the first half (ouch). In addition, the USA shills, FBI, and their proxies launch countless smear campaigns against Yoimiya.
Putting Yoimiya with Nahida signals that the community realizes their past mistakes and are starting to atone for their past sins.
We free Nahida, defeat Scaramouche, learn that Dottore has clones, cleanse the Irminsul (which causes the world to forget Rukkhadevata's existence), and learn that the traveler is the 4th descender of Teyvat (and that their sibling is not a 'descender'). The 1st descender is referred to as the 'Heavenly Principles'. We also learn the Ars Goetia name of the hydro/justice/Fontaine archon, Focalor.
Whoever came up with the names for the 3.2 archon quests and Nahida's story quest must have been raging or malding hard.
The Scaramouche boss' epithet (Everlasting Lord of Arcane Wisdom 七叶寂照秘密主 qiye jizhao mimi zhu) is a combination of all the things that people hate in the world. It combines all the hypocrisy from history. The music slaps hard too. Besides Azhdaha, it is perhaps safe to say that Scaramouche is the best designed boss to date.
While battling Scaramouche, Nahida and the traveler use a dream samsara loop (168 times) to study his combat behavior. They then use the combined strength of the Sumeru people's consciousness in the Neo Akasha Terminal to defeat him.
The Irminsul was contaminated by forbidden knowledge from the abyss. The forbidden knowledge is something hard to understand, even for Rukkhadevata, but is a kind of 'truth' that could not be understood (so something from Godel's Incompleteness Theorem?). The world constantly rejected this forbidden knowledge, leading to all sorts of bad phenomena. (This kind of motif appears in star rail with the stellaron and fragmentum contamination. The ley lines that can be challenged are connected to Irminsul in Teyvat too.)
The forbidden knowledge was what caused a civilization of Sumeru to turn into desert long ago. King Deshret sacrificed himself to save the people from the contanimation. Deshret worked with Rukkhadevata at the time. Forbidden knowledge is also what poisoned Apep and came as part of the cataclysm 500 years ago. During the cataclysm, Rukkhadevata, poisoned by the forbidden knowledge, sacrificed herself to save Sumeru. She created the Akasha terminal to compile the collective knowledge of Sumeru denizens to combat the contamination. She snapped off the purest branch of herself, creating Nahida. The only way to cleanse the forbidden knowledge from the Irminsul was to erase Rukkhadevata from the world as well. This happens, and it is quite sad.
At present, forbidden knowledge still exists in the form of canned knowledge. And this canned knowledge can cause people to go mad. The Withering (which can decrease hp and res) was also a result of forbidden knowledge from Deshret's time. The Eleazar disease is a manifestation of the withering on the human body. After Nahida purges Rukkhadevata and the Irminsul of the forbidden knowledge, the Eleazar on Dunyazard and Collei are cured.
After saving Sumeru, Nahida discontinues the Akasha.
In Nahida's quest, beating up Emira is revenge for all the crimes Emi committed.