Clarifications about Imam al-Mahdi

Recently, I heard that a hadith states that the day of judgement will never be appear before the arrival of Imam al-Mahdi.

Then, I started searching details about Imam al-Mahdi.

I gone through many documents in internet like this.

I also heard some strange things about Imam al-Mahdi from some Islamic scholars personally, like

Imam al-Mahdi had already born just a few years ago(ie, the day of judgement nears us)
Even Imam al-Mahdi does not know he is Imam al-Mahdi.
At the time of His Hajj, a sound from sky will appear which sounds that he is Imam al-Mahdi.(This will happen within a two or three years,Insha Allah)
Muslims all over the world(Sunnis,Shias,and all other muslims) will come under his Khalifath.
Then only Dhajjal will come.

Can anyone explain detail about Imam al-Mahdi and also about the above informations.

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