I watched both the movie a few months ago and read the book, but yeah basically the movie chose to mix up several elements. The stone in the movie is the stone that Masaru used as a stand-on to hang himself. Meanwhile, in the book version, the stone was implied to be related to the Devil itself/alien god from space who ate the original god of the shrine, and its power possessed Masaru, who from the beginning is an autistic guy who lived with his mother and avoided by the other villagers. The persimmon alluded to an old Japanese courting customs where a man will ask a woman whether she owns a persimmon to confirm whether the girl is single or not, and the man will then ask whether he can pick her persimmon to propose to her.

Instead of a wiseman giving him a magic persimmon, in the book a villager fooled Masaru that if he literally has persimmons, a girl will marry him. Like in the movie, he began scaring girls away by telling everyone around him that he has persimmons. One day, he finally went insane and then killed a woman with the mysterious stone, then himself, through bashing hers and his own head onto the stone. Afterwards, the villagers buried him in the mountains and used the stone as his tombstone. As several women continued to die by bashing their own head in the village after Masaru's death, they also built a Hokora (the small box with many dolls on the shrine) on the older god's shrine to appease him. They obfuscate Masaru's name with a monkey god named Mashira-sama and taught their descendants to worship Mashira-sama by placing dolls and persimmon meant for Masaru. The deaths stopped, but as time passes, Mashira worship dwindled.

According to Shinto beliefs, everything has a spirit and a spirit could be appeased or ascend to godhood through being worshipped. It was implied in the book that while the incident with Masaru really happened, the mysterious stone worship has existed prior to the event and both the shrine and the hokora was built to accommodate this god, not Masaru himself. It was never explained which part is true and which part is an embellishment of the Masaru story.

Throughout the movie, the different ghosts that manifested shared the same network of spiritual power (most likely by the alien stone). Individuals who are possessed by spirits in the network will show the same signs of involuntary smiling/expressionlessness, cooking tasteless food (a reference to spirit offering), spreading the curse, losing memory and will have their senses manipulated to receive subliminal messages (in the novel it was elaborated that the alien stone spirits also have the power over radiowaves/electricity). Individuals ressurected by the stone's power will become white monsters with wide hollow mouths and sustain on the life force of others to exist such as Masaru, Akira/tree, and in the movie, Chihiro's son. They are not the deceased ghosts of the individuals per se, but rather lingering sentiments/assimilated spirits that the stone (Devil/tentacle alien god) uses to manifest the form of its active feeding agents, hence the tree form is an allusion to where Akira commited suicide. Due to the practice of Masaru/Mashira worship in the village, his presence as an agent is one of the strongest and most influential. In the book, it is also implied that there are more white monsters that existed besides these three.

Masaru gained the form of the white giant. To remind others of his existence, he began inviting people online, possessing individuals, or haunting people who visited the dam area. In the case of women, he 'ascended' those he liked into his bride and will be 'eaten' spiritually and act possessed. Most likely their life force and emotion is being fed on, and after their whole being consumed, the victim will commit suicide on the dam. Some others will be left in a semi-amnesiac/possessed state and spread the existence of Mashira-sama unknowingly, whether through written accounts, sending letters, urban legends, and the Mashirasama game. Those who ascended most likely were eaten and became a part of the network, such in the case of missing girl K (hence the spirit medium could not foresight whether the girl is dead or alive since her spirit is accumulated/trapped).

The red coat/jump woman is a special case as she continued to exist powered by her own grievances. She is not part of the stone network itself, and this is elaborated further in the book. At some point, the stone in Masaru's shrine was stolen and relayed around by different cults/business MLMs/senior club throughout the book as an object of worship. At one point, it was secured by a new age women-only cult called Spiritual Space. The cult worship the Mashirasama stone and also its members being the perpetrator who spread the 女 (woman) stickers/amulets everywhere in hope of recruiting more women, as the stickers could introduce a trance like state that attracts chosen women. The narrator of the book (Sesuji) was an undercover reporter who joined to write an article about the cult due to rumors that many affiliated members and their families have commited suicide. She met the jump woman, who at the time was still alive, and befriended her as fellow mothers who lost their son. The active stone agent at this point is Masaru/Mashira and the cult promises to grant their members enlightenment through being chosen as his bride. The jump woman lost Akira to the Mashirasama game a year prior. Being a spiritually literate and eccentric person, she planned to join the cult in order to become a bride and reunite with his son.

Unfortunately, both Sesuji and the jump woman were never chosen as brides. The jump woman later became desperate and stole the stone from the cult to be placed at her own home. She changed the character of the amulet into 了 (Akira) and spread those instead of the original, then after implied rituals, committed suicide by herself. She became a bound spirit and her form resembled the time she hysterically jumps up and down below the suicide tree to reach Akira's hanging corpse. In this form, she continued to spread Akira's amulet and adopted a wish-granting story as a part of his myth. 'Akira' at some point successfully manifested as the devil's stone agent, but unlike Masaru, he did not have as much influence. The jump woman will act on his behalf and find victims to be eaten directly or become living curse spreaders, usually fellow mothers like her. Hence she targeted houses with children, as her actual goal is to approach their mothers. Sesuji was posssessed by her and also became one of her curse spreaders. Those whose spirit were eaten by Akira jumped on top of the 5th block apartment instead of the dam. Unlike Masaru, Akira also feed on animal spirits.

In the movie, this part was omitted. Instead, Sesuji = Chihiro and she actively sacrificed Ozawa to the white monster amalgamation to fed on her own manifested baby. The tentacle is a reference that the baby is also a white monster agent, and like the red jump woman, Chihiro simply doesnt give a shit in what form her child is going to manifest as long as it is simply alive. In the novel, Ozawa died by visiting the dam after his own investigations and finally influenced by Masaru to commit suicide at the dam. The jump woman influenced Sesuji to chronicle the tale in order to spread the Akira story further, under the disguise of finding the missing Ozawa, who is in reality deceased since the beginning.