I'm not sure but I'd say it was probably done with 2d bone animation. Something like Anima2D from Unity Asset Store. It hasn't been on there for at least 5 years as far as i know but I believe there are alternatives to it with similar features.
Than again, I might be as wrong as one can be, so don't take my assumptions as a fact
its a 3d model(just modeld the flat visibile areas) and the grow is animated in this model. with the help of unity shaders it look from stage to stage very smooth
