FBS doesn't contain cells. It's a chemical/protein mixture used to grow various cell cultures. On It's own it doesn't do anything. It just contains most of the things that growing cells need.

I actually know very little about it though. Check out The Thought Emporium on youtube if you want a better look at how it is all done. They do all sorts of fun stuff like engineering yeast to produce spider silk, growing neurons and using them for basic computation, making a meat berry, and using a genetically engineered virus to cure their lactose intolerance for a while.

Edit: Also that youtuber did some testing on replacing FBS and the growth medium with other substances with some sucess. If I remember correctly, they found that for growing regular muscle cells they could replace like 60% of the FBS needed with egg yolk without harming the cells. But that's still an animal product so it's still not exactly vegan.