Where do biologists get information about mouse genes?
Hi Potion and welcome to Biology Stack Exchange and biology in general!

To answer your questions, these changes do not occur. The authors of the publication have engineered "new" genes (also called transgenes in this context). 

They have done so by fusion a portion of a of another gene (also called domain) to the beginning or the end of the sequence of the catalase gene. If your gene of interest is localized to a given organelle (for example mitochondria), you can also swap the domain which is responsible for the targeting to this particular organelle with one from a different gene you know goes for let's say peroxisomes.

If you want to know more details for this particular work I encourage you to check the supplementary material here: https://science.sciencemag.org/content/sci/suppl/2005/06/23/1106653.DC1/Schriner.SOM.pdf

Hope that helps!