How to save modifications before theme update w/o child theme

Is there a way to save my modifications to a theme without a child theme? Making a new child theme only saves it as that current theme’s version, right?

I’ve made a child theme before, which wasn’t a big deal. But it doesn’t apply the update. So I have to apply the update then switch to the parent theme and edit it that way if I want the theme’s newest updates.

Does any of that make sense? hah.

site: https://therichstyle.com and I’m using the Brunelleschi theme.