First of all, there are 15+ Immortals in Sanatana Dharma, but they are not properly mentions because there are many ancient text proofs are destroyed and lost by time.
The seven immortals were from any different timelines, even some of them are alive from many lakhs of years.
Here is the list of Seven Immortal:
1. Lord Parashurama
Parshuram Lord Vishnu’s sixth incarnation is Parasuram.
Lord Parasurama Ram was back, but he was also in Ram’s time due to obtaining a Chiranjeevi ( Immortals ).
Parshuram had 21 times the end of all the Kshatriya kings from the earth.
He goes regularly Yamlok also for his guide and for attending a special conference of Dharamraj and some Rishi Munis there. Yamraj always calls him in case of any complex matter of Jivatma Karma.
Bali The talk of donation of King Bali was far and wide. King Bali took control of IndraLok to climb up the deities.
He would be as Heaven God Indra in next Manvantar as per Lord Vishnu wish and assurance.
The sacrifice took place during the time of Lord Vaman Avatar in Satyuga. In order to destroy the pride of King Bali, God had asked for three donations of land from King Bali by donating the Brahmin’s front.
King Bali said that where you wish, keep three feet.
Lord Hanuman can be seen with such devotion and Bhakti only.
Common people who are not pure as per Hindus culture were run away from his presence in Kalyug.
One of the brother of Ravan, Vibhishan, who fought on Ram’s side in the Ramayan.
He was made a Chiranjeevi to preserve morality and worship in Lanka and to guide people over the world in Dharma.
He is visible also by only divine eyes alone. Goswami Tulsi Das had seen him about 500 years backwards in Kalyug.
5. Ved Vyas
The person who narrates Mahabharat to Lord Ganesh. He split the Vedas into 4 parts ( remember he didn’t invent Vedas, he actually wrote the Vedas into the simplified form ).
Kripa was the Kul Guru of the Kurus during the Mahabharat, including whilst his status as a Chiranjeevi is disputed, his neutrality towards all of his students is the most commonly documented reason for his immortality. He achieved the knowledge and power of cosmic energy.
Aswasthama was The curse by Lord Krishnam, that’s why he is immortal.
He was seen by several peoples in India. I have met someone in Bihar, UP and even Gujrat, who was firm that he had seen Aswashthama.
He was the son of King Bali who was king of asuras.
He was an ardent devotee of lord shiva.
Banasura had thousand of hands. He was deeply devoted to Lord shiva. Even Shiva fight against Lord Krishna to rescue her Daughter from Grandson of Lord Krishna.
He was a very famous character in Ramayana and even he also caught in different times of Lord Rama, Lord Krishna and Bali.
According to Ramayana, he was created by Brahma. He is an immortal entity.
Some of them are immortals and die at the end of Kali Yuga that is Hanuman, Bibhishan and Baali they are blessed by Lord Vishu.
Even few of them have knowledge and power of cosmic energy from this they achieve the long live Superhuman body and mind that is Kripa, Ved Vyas