Please tell me about the seven immortals in Hinduism? How did they become immortals?
As mentioned here
Chiranjivins (Sanskrit nominative sing. ciranjivi, चिरंजीवी):
The word is a combination of "chiram" (long) and jivi (lived).
Aswathama Balir Vyaso Hanumanash cha Vibhishana Krupacharya cha Parashuramam Saptatah Chirjeevanam
Which means that Aswathama, King Mahabali, Veda Vyasa, Hanuman Ji, Vibhishana , Krupacharya and Lord Parashuram are death-defying or imperishable personalities.
Markandeya, a great rishi and Jambavan are also considered as Chiranjivins.
Brief note on Chiranjivins:
Aswathama did not attain the so called Chiranjitvam due to any noble deed. It was a curse from Lord krishna after Aswathama killed all the sons of Pandavas with his Brahmastra.
Mahabharata, Book 10: Sauptika Parva, Chapters, 15 & 16: Aswathama could not take back Brahmastra and with that killed the sons of the Pandavas. But Lord Krishna gave rebirth to the foetus, that of Parikshit.
Lord Krishna cursed Aswathama as follows;
Always engaged in sinful acts, thou art the slayer of children. For this reason, thou must have to bear the fruit of these thy sins. For 3,000 years thou shalt wander over this earth, without a companion and without being able to talk with anyone. Alone and without anybody by thy side, thou shalt wander through diverse countries, O wretch, thou shalt have no place in the midst of men. The stench of pus and blood shall emanate from thee, and inaccessible forests and dreary moors shall be thy abode! Thou shalt wander over the Earth, O thou of sinful soul, with the weight of all diseases on thee."
King Bali is immortalised because even Vishnu had to humble himself as Vamana to conquer him. He stands out for his valour and charity — two great eternal qualities. Bali knew fully well who Vamana was. Yet, he prepared to surrender all he had, to his Lord. So this man of grace is still revered.
Vyas, the great scholar and writer, the author of Mahabharata and the Srimad Bhagavatam, is another immortal, a shining beacon of erudition and wisdom. A katha-wachak reading from the holy texts, is said to ascend the Vyaspeeth — blessed by the immortal Vyas. He comes to represent the continuity of erudition, scholarship and the apotheosis of the writer as the ultimate visionary.
Hanuman exemplifies selflessness, courage, devotion, energy, strength and righteous conduct. He stands for the potential that is inherent in all of us. He also represents the air, the atmosphere and thus the life-breath — the prana. Sita is believed to have bestowed on him the boon which made Hanuman ever-youthful, energetic and immortal.
Vibhisana had unshakable faith in what he believed. He was a fearless counsellor of righteousness even when all around him were given to sycophancy and peer pressure. As a follower of right conduct, Vibhisana survived unscathed while tragedy befell the unrighteous king and his followers.
Kripacharya was an extraordinary teacher. To him, all pupils were equal. A guru like Dronacharya favoured Arjuna and sacrificed an equally if not more promising student — he asked Ekalavya for his right thumb. Kripacharya, on the other hand, upheld the highest standards expected of a teacher: He was impartial.
Parashuram was master of martial arts. No one could beat him — whether the tools were astras, sastras or divine weapons. He had no temporal ambition. Had he so wished, he would have been the greatest Chakravarti Samrat India had ever known. But he lived a hermit's life. He was also one who never hesitated to admit a mistake.
During Sita's swayamvar, after Ram had broken the divine bow of Shiva, Parashuram asked both Ram and Lakshman for forgiveness. Despite his excellent qualifications, he did not hesitate to apologise in all humility that he had lost his reason in anger. Parashuram symbolises excellence and strength, tempered by humility.
There are many stories about immortal people in Hindu Epics
Ashwathama, King Mahabali, Vyasa, Hanuman, Vibhishana, Kripacharya, and Parashurama are the seven death-defying or immortal personalities. They are also called Chiranjivis.
The extant Puranas, the Ramayana, and the Mahabharata describe seven long-lived personalities in the Hindu pantheon. While there are others as well, which are not included in the one particular shloka.
"Ashwatthama Balir Vyaso Hanumanash cha Vibhishana Krupacharya cha Parashuramam Saptaita Chiranjeevanam"
The Chiranjivins are as follows:
Ashwathhama: There's an incident in Mahabharata in which Yudhishtir says "Ashwathham hato, naro va kunjara" to distract Dronacharya, which means, Ashwathham is dead, either human being or animal. At that time, an elephant named Ashwathama died, while the son of Dronacharya is still alive. He killed Upapandavas by misinterpreting them as Pandavas and later, he tried to kill the Abhimanyu’s (Arjuna’s son) wife Uttara. For these wimpy acts, Lord Krishna cursed him to remain alive forever and bear this burden i.e. Ahswathhama is still alive.
Vibhishana: Vibhishana, one of the most conscientious, righteous and ethical characters in Hindu mythology. He was not only the youngest brother of Ravana but wisest too. He never wanted a battle between Rama and Ravan and thus advised his brother (Ravana) to not to keep Sita (Rama’s wife) as captive. He asked him to return Sita to Lord Rama and apologize. Instead of listening to his words, Ravana humiliated him. Thus he joined Rama’s force and revealed Ravan’s death secret. For his noble, wise and ethical act, Lord Rama acknowledged him with immortality.
Kripacharaya: Kripacharya was the son of Shardwan and Janpadi and used to teach the royal children, Kauravas and Pandavas. Prior to appointment of Dronacharya as a teacher by Bhishma, he taught Kauravas and Pandavas about ethics, war strategies, administration etc. He is also one of the chiranjivis. He was also appointed as a mentor of Arjuna’s grandson Parikshit, and due to his dedicated, devoted, determined and loyal behaviour, he earned immortality till Kal Yug (i.e. present age).
Hanuman: Next name in the list of Chiranjivis is of Pavan Putra Hanuman. There are religious songs like "Shri Ram Janki, baithe hai mere sine me" and "Ram ji chale na Hanuman ke bina", which clearly shows the love and affection of Hanuman towards Lord Rama. He was utterly selfless and scarifying and left no stone unturned in helping Lord Rama. He did everything, whatever he was capable of; let it be a search of Sita, or getting the Sanjeevani for Lakshaman. As a result, Sita granted him immortality, for spreading the name of Lord Rama across the globe.
Parshuram: Known for his anger, Parshuram is the next chiranjivi. He is one of the anshawatar of Lord Vishnu (sixth avatar) and known as a guardian angel, savior of mankind. History does not mention anything about his death and it is believed that he will reappear whenever mankind will need him. He was believed to be the master of weapons and arms, and there is also a belief that he will re-emerge for atonement. He is the fifth immortal.
Bali: Bali Chakrawarthi was one of the most powerful rulers in Hinduism ever. He is known for his strength and courage, and the myth even says that he had the power to acquire half of the strength of his opponent. He conquered the whole world through his fighting skills and bravery. There is an incident when he was performing Ashwamedh Yagya and Lord Vishnu intervened. Vishnu asked him to give back all his land in leu of immortality and from there, he became chiranjivi. Not only immortality, but he was also blessed with the supreme position of heaven
Vyasa: The last name in the list of Chiranjivis is Rishi Vyas. He is the creator of many Puranas and others. He is wisest of all and well known for his knowledge. He was born in late Treta Yuga and was the storyteller of Mahabharata. It is believed that he retreated in some remote areas of North India. Vyasa is one of the most intelligent characters in Hindu mythology.
Actually there are 8 immortals in Hinduism, they are:
Rishi Ved Vyasa.