How do you get rid legacy system extensions?

See this for more info: About legacy system extensions - Apple Support

You will need to use an uninstaller or uninstaller instructions provided by the vendor. These things usually cannot be removed by dragging an app to the trash.

In an ideal world, when you research where this software came from, you will find that the vendor provides an updated version of the software in question. More likely, you will find that the vendor has abandoned the software. In some cases, there may not even be an uninstaller available. You may have to ask here for help in removing it. Or, if you have more than one of these software packages that you can’t properly remove, it may be time to erase your hard drive and do a complete reinstallation.

You don’t need to make any changes at this point. But it would be an excellent idea to take Apple’s advice and start doing research. Do you really need this software? If so, for what? Are there any alternatives that don’t use these kernel extensions? You do need to do anything now to export your data into more portable formats while you can still use the software?

After loading updated OS Catalina 10.15.6 last night, I received the warning about legacy extension incompatibility. I clicked on "learn more" about legacy extensions, but then original message went away. Now I don't know how/where to identify the incompatible app or developer that was discovered on my iMac. How do I find this again, so I can contact them?

For now, you don't.

How do I get rid of Legacy extensions

How do I get rid of Legacy extensions

What did you not understand from the previous replies that explained what to do?