Sandun found Penny maiden name.
However, during a good part of the story, before she marries Leonard, her legal name was Johnson.
Indeed she did marry Zack Johnson in Las Vegas, and even though both thought the wedding was just for fun, the marriage was in fact perfectly valid. This was told in The Thanksgiving Decoupling, season 7 episode 9. The wedding ...
There are really a lot of references to BBT in Young Sheldon. One misses a lot of the fun if one is not familiar with Sheldon's idiosyncrasies.
One sees how they started. If one does not know them, the joke is lost. So though it is not absolutely necessary, I would strongly recommend watching at least a good fraction of BBT first.