Dreamweaver cannot compare text files (such as code) itself but it allegedly allows you to use certain third party file comparison apps to do it by using the Dreamweaver interface.
BUT there are none that I know of that work with Dreamweaver on either macOS and Windows (as of 13 July 2021). Note that it used to work for me. I believe I used the now unavailable TextWrangler app.
Here is the link to the Adobe site's Dreamweaver information about file comparisons (as of 13 July 2021). Unfortunately it has not helped and note that their info has not been updated in quite a while AFAIK.
According to that page in the Dreamweaver User Guide at the Adobe site, one would first need to download a file comparison app that will work. If you can find let me know. Then you would use Dreamweaver Preferences > File Compare to select the third party comparison tool that you have decided to use and already have installed on your computer.
Then you simply select from the Dreamweaver Files panel (by using Ctrl or ⌘-click) the two files you want to compare. Then right-click on either one of the two files and select Compare Local Files from the menu that appears. Next, the third party tool you specified ought to open up with those two files side by side with their differences indicated in some way, depending on the tool.
Again, the problem is that there are no third party tools that I know of that will actually work. The Dreamweaver site only recommends these three file comparison tools - with my comments:
Note that there are some online comparison tools, but using them is not convenient and requires cut and paste or reopening the file(s) through the web page UI in order to work rather than simply saving any new changes and refreshing the comparison as one would expect to be able to do for this type of work.