What happens the value of your shares if the company turns into a parent company?

Newbie question, but for example, if you bought Tesla stock, and at some future date Tesla turned into company “X” which owns Tesla, SpaceX, Neuralink etc. What happens to the value of the shares you own? Do you get any “free value” from the incoming subsidiaries?

I imagine something similar happened with Facebook/Instagram -> Meta, Google etc -> Alphabet but I might be wrong.