Per Live Science, the team modified a technique used to turn stem cells into neural tissue to create the brain organoids with the optic cups. After the cells developed into mini-brains, the optic cups formed 30 days later and fully matured at 50 days. The timing was on target with eye development in a human embryo, meaning the process could be used to study how eyes develop in utero.

In the past few years, it has become possible to encourage stem cells – versatile cells similar to those found in embryos – to grow into spherical masses of brain tissue up to three millimetres wide, known as brain organoids.

https://www.newscientist.com/article/2287207-tiny-human-brain-grown-in-lab-has-eye-like-structures-that-see-light/

Every time certain segments of society speak it causes small potions of the audience's brains to drip out of their ears. These bits are scraped up off the floor and, after mixing well with some tapioca, come to plausibly resemble the mental functions of the person last heard speaking.

Radiolab did a great episode on it recently called "Brain Balls"

https://radiolab.org/podcast/brain-balls

https://www.youtube.com/watch?v=bEXefdbQDjw

I assume it's not that different from growing a rat neuron blob.