Updating modified templates

I want to tweak some of the .php template files in my WordPress theme, and I understand that the correct process is to copy the relevant files into a child theme folder and edit the files there, so that my modifications won’t be lost in future theme updates.

But does that mean that I then won’t get the benefit of theme updates to the files I’ve put in my child theme folder?

This might not be a good thing, as the theme update may have added some useful features, or even fixed the issues for which I initially needed to make the code tweaks!

What’s the common solution to this? Someone suggested a Diff app – is this what people commonly use?

Yes, it’s about to check changes after updates of main theme, mostly updates goes to core of theme, not files like single.php etc.. but when it’s go this way, you need some Diff program. Check this out: https://stackoverflow.com/questions/25874018/comparing-the-contents-of-two-files-in-sublime-text