Biggie's rhymes weren't freestyles. They were thought of before hand. You nikkas gotta stop being so gullible. No human alive is freestyling those records big did. You dont have to store rhymes on paper, you have a brain that u can use as well.
It's simple actually. Just think of a line or two. Memorize it. Rinse and repeat till u get to 16 bars then repeat the process over again for the other verses. It's also easy to do this while recording, because you can punch in after a line or two and build it all the way up to 16 bars or wateva you want for each verse.