new functions in child theme

My theme is using a theme-functions.php and I am working with its child theme. I have created a custom menu and addeded it to the code that was inside that theme-functions.php file. This is unsave in case of an update. How exactly can I save my custom menu? I know, there is a way to create a functions.php file for the child and in my case, it would mean that I have to create a theme-functions.php, right? However, I do not see what part of code I would have to put in there because it supposed to load before the original theme-functions.php, right? That would mean, my file would be overwritten. This is confusing. So what is the challenge for me, how can I save my custom menu in case of a theme update?