The way we grew them normally was in those roller bottles [2]: you have some tissue culture medium in the bottle (containing fetal calf serum, which is still required to grow mammalian cells AFAIK, another reason most of these companies can't do what they say) and other components. Then the bottles are put on their sides on a roller mechanism, in an incubator with a controlled CO2 concentration.
The cells want to grow in a one-cell-thick monolayer all over the inside of the bottle. What they won't do is to grow a double layer. This is because they have "contact inhibition" which is a major mechanism for preventing tumor-like growth: when cells are touching each other, they stop dividing.
So these guys are trying to grow tissue and what they seem to be doing instead is growing monolayers just like this on the inside of roller bottles, scraping them off, then putting them together again.
One thing that disturbed me was that they need to coat the bottles with porcine gelatin. In that case, a lot of those "chicken" cutlets are actually pork!
[1] WI-38 cells: https://en.wikipedia.org/wiki/WI-38
[2] Tissue culture in roller bottles (yes, the medium is red at least for mammalian cells): https://cdn11.bigcommerce.com/s-hgsfepq18i/images/stencil/12...