Pain (Nagato) got the Rinnegan because Madara Uchiha secretly transplanted his own Rinnegan eyes into young Nagato as a child, intending for Nagato to revive him later; Nagato's Uzumaki lineage with strong life force allowed him to handle the power, awakening them fully in grief after his parents' death and later using them through the Six Paths of Pain, which are projections of his power.
Nagato's Rinnegan was implanted by Madara when he was child. When Nagato uses the Six Paths of Pain, the Rinnegan is reflected in the eyes of the bodies that he uses as a side-effect so Pain appears to have the Rinnegan.
As with any dōjutsu, the Rinnegan can be transplanted into others. Prior to his death, Madara implanted both of his eyes into the young Nagato without the boy's knowledge, and he was able to make effective use of them throughout his life due to his Senju lineage.
Madara planted his Rinnegan into Nagato, an Uzumaki survivor of the Hidden Rain Village. Although we do not know when he implanted his Rinnegan into a young Nagato, we know it must've been during the time of the Third Great Ninja War. Pain/Nagato has only one pair of Rinnegan.
Nagato, also known as Pain, received his Rinnegan eyes from Madara Uchiha. After the death of Nagato's parents during a conflict in the Hidden Rain Village, he was approached by Madara, who recognized Nagato's potential.
Even though it wasn't explicitly shown, it's also widely believed that he used Izanagi after Sasuke woke up and he was caught in Amaterasu thanks to Itachi planting them, meaning he also had 2 sharingan back before the Konan fight.
No, Madara gave Nagato his eyes before he met Obito. Madara only had a single Sharingan at that time because his true eyes were "on loan".
Sasuke left Konoha (the Hidden Leaf Village) in the original Naruto series primarily to gain immense power from Orochimaru, fueled by his all-consuming desire for revenge against his brother Itachi for massacring the Uchiha clan, feeling the village held him back and his attachments weakened him. He was driven by trauma, a feeling of inadequacy, and a desperate need to surpass Itachi to avenge his family, believing Orochimaru offered the fastest path to that strength.
A descendant of the Uzumaki clan like Naruto, Nagato's Rinnegan is later revealed to have belonged to Madara Uchiha, who transplanted his eyes into the child as part of his plan. Nagato desires peace for the world, helping Yahiko to create the Akatsuki to achieve that shared dream.
In the Naruto universe, Rinnegan individuals may access an Outer Path technique called the Six Paths of Pain. With it, they may exert influence over as many as six bodies as if they were their own.
Rinnegan requires a complex fusion of chakra and lineage in its active maintainence. In Sasuke's case, having been given only half of Hagoromo's chakra only allowed for the Rinnegan to awaken in one eye. Madara and Obito also faced this kind of problem.
When Nagato is brought back with Edo Tensei, he has the Rinnegan, but not his original eyes. Maybe it's because he died with those eyes. But when Madara is reanimated, he has his original eyes, even if he supposedly died with different ones.
He revealed later that he simply couldn't handle the chakra strain from both, barely handling one Rinnegan.
The first person on Earth to wield the Rinnegan was Hagoromo Ōtsutsuki, whose mother possessed the Rinne Sharingan. In the anime, he obtained his Rinnegan at the same time as his Mangekyō Sharingan, which awakened as a result of mortally wounding his brother.
1. Naruto Uzumaki. Naruto Uzumaki, the Seventh Hokage of Konoha, has grown from an underdog to one of the most powerful shinobi in history. As the host of Kurama, the Nine-Tails, Naruto possesses immense chakra reserves, which he can tap into to access Kurama's power.
The Beginning of the Crimson Spring (紅き春の始まり, Akaki Haru no Hajimari, Viz: The Start of a Crimson Spring) is volume 69 of the Naruto manga.
One of Nagato's parents was a scattered Senju, the other was a scattered Uzumaki.
While he doesn't explicitly name himself as genderfluid, this description surrounding Orochimaru's gender had Naruto fans rejoicing that the series confirmed its first LGBTQ character.
Kishimoto: Naruto didn't become a jounin. He became Hokage as a genin.
Madara fighting while blind.
Linking with the Demonic Statue of the Outer Path left a number of black receivers embedded in his back and caused his body to become very emaciated, limiting his mobility. For this reason he spent most of his adult life in a mechanical walker from which he controlled the Six Paths of Pain.
He looked up to his older brother and was so close to Madara that he even gave him his eyes as a dying gift as a means of ensuring that Madara will be able to better protect the clan's future.