Madara:
Fights 1st hokage
Gets away
Acquires Senju DNA
Uses that DNA to create Zetsu
His goal was to obtain the senju powers.
He learned that Zetsu has crazy healing\restoration powers cuz of the senju power.
Madara repaired himself as much as he could using the senju power.
Madara used Zetsu and tried to revive Izuna.
Izuna's body was re-animated but he was not able to pull the soul from the afterlife.
So Black Zetsu was created from Izuna corpse + White Zetsu.
White Zetsu is purely the senju power and that's why it is mass produce and black zetsu isn't...
I think that the Black Zetsu is Izuna (might not know he is Izuna) because:
-He can record battles, seems like a sharingan power to me <Maybe even his MS ability because he is able to share the fight with Tobi\Masked man>
-He claims to be "made of the Earth" - A corpse buried in the ground starts becoming earth (circle of life) as well as White Zetsu being made from senju DNA which seems to have earthly characteristics and qualites.
White Zetsu is mass produced and nothing more than senju's powers being manipulated and controlled by Tobi\Madara
This also explains the two parts of Zetsu splitting, and 2 personalities. Perhaps the black zetsu shows similar character traits to Izuna.
Madara lived a long time. I think he was alive when Nagato was born, but was close to dying.
I think Madara created Tobi using white Zetsu and his body (after his death) in order to carry on his moon's eye plan.
I believe Madara's goal was to go to the afterlife and trust his Tobi creation and Black Zetsu to carry on his will.
Madara unlocked the rinnegan close to or soon after Nagato's birth.
Since Madara was old and severly weakened from his battle with the 1st, he wasn't able to advance and use the rinnegan as much as he wanted to.
His work around was plant the eyes in Uzimaki Nagato (good choice in Uzimaki, and Tobi has complimented the Uzi clan many times)
Tobi would manipulate Nagato as well as monitor Nagato's growth and usage abilities with the rinnegan.
His ultimate plan was to be revived by Nagato later with a newer stronger body.
At this point Madara and Izuna's souls are both in the afterlife.
Tobi is Madara's body and most of his will - living because of Zetsu powering (How many times has Tobi said he's a shell of his old self

, body with no soul = shell)
Zetsu is Izuna as explained earlier.
As we all know, the plans of being revived by Nagato failed.
When Madara first came back, he instantly noticed something wasn't right. Of course that was because he was revived by Kabuto.
I think Madara was suprised because his body was in it's prime as far as health\strength, yet he also had the rinnegan.
I'll try to stop now, love writing.
AS ALWAYS:
THIS IS A LOGICAL GUESS.
MEANING IT IS AN OPINION.
IF YOU HAVE HARD PROOF FROM A LEGIT SOURCE THAT SOMETHING HERE IS INCORRECT, PLEASE LET ME KNOW!
I would update my guess and thank you for pointing out something i overlooked\forgot about etc.
IF YOU DISAGREE WITH THIS, THAT'S FINE.... THAT'S THE BEAUTY OF OPINIONS AND THEORIES - THERE CAN BE MANY!
Other than that,
N~joy and sorry if this is double post, It didn't seem to be @ a glance.