Upon the consensus of the sahabah
Upon instruction of the Prophet ﷺ, according to the guidance from Allah ﷻ . This is stronger opinion, due to the following hujjah:
By looking at the ending of a chapter and the beginning of the other chapter, the synergy of the chapters is a phenomenon.
It is تناسب tanaasub which means there are connectivity between the surah e.g. Al-Fatiha ending about Allah's anger towards the Jews ٱلْمَغْضُوبِ , and Al-Baqarah that follow suit provides the tafseer of ٱلْمَغْضُوبِ while Ali Emran explains the tafseer of ٱلضَّآلِّين.
Al-Quran was revealed in two stages:
In one go from Bayt al-Ma'moor to the first Heaven
From the first Heaven to the earth over a period of 23 years, in piecemeal
Allah ﷻ says in Surah Al-Baqarah verses 185:
شَهۡرُ رَمَضَانَ ٱلَّذِيٓ أُنزِلَ فِيهِ ٱلۡقُرۡءَانُ هُدٗى لِّلنَّاسِ وَبَيِّنَٰتٖ مِّنَ ٱلۡهُدَىٰ وَٱلۡفُرۡقَانِ
Ramadān is a month in which the Qur’an was sent down as a guidance for mankind and as clear signs that show the right way and distinguish between right and wrong. [QS 2:185]
And also in Surah Al-Qadr:
إِنَّآ أَنزَلۡنَٰهُ فِي لَيۡلَةِ ٱلۡقَدۡرِ
Indeed, We sent this [Qur’an] down on the Night of Decree. [QS 97:1]
Ibn ‘Abbas (رَضِيَ ٱللَّٰهُ عَنْهُ) said concerning the verse “Verily! We have sent it (this Quran) down on the night of Al-Qadr (Decree)”:
The Quran was sent down all at once on Layl al-Qadr, then Allah sent it down to the Messenger of Allah (ﷺ) bit by bit. “ And those who disbelieve say: Why is not the Quran revealed to him all at once? Thus (it is sent down in parts), that We may strengthen your heart thereby. And We have revealed it to you gradually, in stages (QS 25:32).” [an-Nasai]
This status of this hadith is marfoo' (means: raised) where the chain goes back to the Sahabah, though it didn't connect direct to the from the Prophet ﷺ, the means the Companions received it directly from the Prophet ﷺ, hence the hadith is being raised, marfoo'.
Verses of the first revelation
The first verses to be revealed were the first 5 verses of Surah al-‘Alaq, when the Prophet ﷺ became the Prophet of Allah ﷻ. Then, there was a pause in revelation. Surah al-Muddassir was revealed after that which marked the Prophet ﷺ became the Messenger of Allah ﷻ. There was no contradiction between Aishah RA hadith (Al-'Alaq) and Jabir RA (Al-Muddassir).
The revelation was paused twice where the first time was after Surah al ‘Alaq was revealed, and the second time was just before Surah ad-Duha was revealed (after 8 surahs were revealed).
Reasons of the verses revealed in the Quran
Quran was revealed due to two reasons which are:
Ibtidaa’i – where the Quran being revealed unconnected to any reason; most of the Qur’an was revealed in this way
Sababi – where the Quran is revealed due to specific reasons, events, questions, an occurrence needing clarity, the need for an Islamic ruling etc. Although the revelation in this way was based on the specific nature of an incident, the guidance revealed is to help public understanding in a general sense.