Matsya avatar is known as the first avatar of lord Vishnu and Varaha comes after that but Varaha came in First Manvantar while Matsya came in seventh. Can someone explain.
This chapter of the Srimad Bhagavata Purana gives the order of Vishnu avatars.
SB 1.3.6 — First of all, in the beginning of creation, there were the four unmarried sons of Brahmā [the Kumāras], who, being situated in a vow of celibacy, underwent severe austerities for realization of the Absolute Truth.
SB 1.3.7 — The supreme enjoyer of all sacrifices accepted the incarnation of a boar [the second incarnation], and for the welfare of the earth He lifted the earth from the nether regions of the universe.
SB 1.3.8 — In the millennium of the ṛṣis, the Personality of Godhead accepted the third empowered incarnation in the form of Devarṣi Nārada, who is a great sage among the demigods. He collected expositions of the Vedas which deal with devotional service and which inspire nonfruitive action.
SB 1.3.9 — In the fourth incarnation, the Lord became Nara and Nārāyaṇa, the twin sons of the wife of King Dharma. Thus He undertook severe and exemplary penances to control the senses.
SB 1.3.10 — The fifth incarnation, named Lord Kapila, is foremost among perfected beings. He gave an exposition of the creative elements and metaphysics to Āsuri Brāhmaṇa, for in course of time this knowledge had been lost.
SB 1.3.11 — The sixth incarnation of the puruṣa was the son of the sage Atri. He was born from the womb of Anasūyā, who prayed for an incarnation. He spoke on the subject of transcendence to Alarka, Prahlāda and others [Yadu, Haihaya, etc.].
SB 1.3.12 — The seventh incarnation was Yajña, the son of Prajāpati Ruci and his wife Ākūti. He controlled the period during the change of the Svāyambhuva Manu and was assisted by demigods such as His son Yāma.
SB 1.3.13 — The eighth incarnation was King Ṛṣabha, son of King Nābhi and his wife Merudevī. In this incarnation the Lord showed the path of perfection, which is followed by those who have fully controlled their senses and who are honored by all orders of life.
SB 1.3.14 — O brāhmaṇas, in the ninth incarnation, the Lord, prayed for by sages, accepted the body of a king [Pṛthu] who cultivated the land to yield various produce, and for that reason the earth was beautiful and attractive.
SB 1.3.15 — When there was a complete inundation after the period of the Cākṣuṣa Manu and the whole world was deep within water, the Lord accepted the form of a fish and protected Vaivasvata Manu, keeping him up on a boat.
SB 1.3.16 — The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.
SB 1.3.17 — In the twelfth incarnation, the Lord appeared as Dhanvantari, and in the thirteenth He allured the atheists by the charming beauty of a woman and gave nectar to the demigods to drink.
SB 1.3.18 — In the fourteenth incarnation, the Lord appeared as Nṛsiṁha and bifurcated the strong body of the atheist Hiraṇyakaśipu with His nails, just as a carpenter pierces cane.
SB 1.3.19 — In the fifteenth incarnation, the Lord assumed the form of a dwarf brāhmaṇa [Vāmana] and visited the arena of sacrifice arranged by Mahārāja Bali. Although at heart He was willing to regain the kingdom of the three planetary systems, He simply asked for a donation of three steps of land.
SB 1.3.20 — In the sixteenth incarnation of the Godhead, the Lord [as Bhṛgupati] annihilated the administrative class [kṣatriyas] twenty-one times, being angry with them because of their rebellion against the brāhmaṇas [the intelligent class].
SB 1.3.21 — Thereafter, in the seventeenth incarnation of Godhead, Śrī Vyāsadeva appeared in the womb of Satyavatī through Parāśara Muni, and he divided the one Veda into several branches and subbranches, seeing that the people in general were less intelligent.
SB 1.3.22 — In the eighteenth incarnation, the Lord appeared as King Rāma. In order to perform some pleasing work for the demigods, He exhibited superhuman powers by controlling the Indian Ocean and then killing the atheist King Rāvaṇa, who was on the other side of the sea.
SB 1.3.23 — In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarāma and Lord Kṛṣṇa in the family of Vṛṣṇi [the Yadu dynasty], and by so doing He removed the burden of the world.
SB 1.3.24 — Then, in the beginning of Kali-yuga, the Lord will appear as Lord Buddha, the son of Añjanā, in the province of Gayā, just for the purpose of deluding those who are envious of the faithful theist.
SB 1.3.25 — Thereafter, at the conjunction of two yugas, the Lord of the creation will take His birth as the Kalki incarnation and become the son of Viṣṇu Yaśā. At this time almost all the rulers of the earth will have degenerated into plunderers.
Some of these avatars are Avesha avatars, which means that the body just has Paramatma and the Jiva inside, but the Paramatma (Vishnu) takes most of the control.
And some of these avatars are complete avatars, such as Rama and Krishna, which means that the body doesn't have a Jivatma, only Paramatma, and Vishnu is in complete control.