As others have said, different stories for so many.

Some not mentioned here that I remember off the top:
-Mustard started as a DJ and made beats for years until his beats got better.
-Mike Will persistently showed up to studio sessions, pretty much had the craziest ambition to be successful.
-Wondagurl and FrancisGotHeat went the beat battle route. Wondagurl had a supportive mother and apparently is naturally super smart. FrancisGotHeat lied to his parents but had a musical background.
-Ant Chamberlain did the type beats thing after being in the Navy. Learned how to produce on the ship.
-Tay Keith was in college so while pursuing a degree meaning he had a place to make beats.
-LilCC was getting paid as a model (probably bank) so she could support herself making beats.

A lot of these producers were self-taught. Some no doubt had musical upbringings, some got natural talent and were undeniable (it is a thing like having absolute pitch), others worked real hard. One thing I don't see mentioned often is how many producers were actually financially stable and did what it took for them to provide a roof over their head while they chased their dream. Others like Mike Will who was getting good grades while in college probably earned their parents' trust to pursue their dream and had good fallback options.

Fallback option is probably the safest way to grind.