I am running Mac OS X 10.15 (Catalina) and the EV3 software can't detect my NXT 2.0 using USB.
LEGO officially stopped supporting NXT in 2015, however, as of 2019, the original NXT programming software is still available on the official LEGO MINDSTORMS web page.
The original NXT programming software and EV3 desktop programming software are both 32-bit, so cannot run on macOS 10.15 or later.
The new EV3 programming software based on Scratch does not work with NXT. So the only way to program NXT on macOS 10.15 is with 3rd party software such as NXC/NQC or leJOS or by installing a macOS or Windows virtual machine to run the old desktop software.
The NXT is no longer for sale by Lego, and I don't think they offer technical support for the NXT platform anymore. However, the EV3 software does support NXT. If you can't connect the software to the brick, it might be a driver issue. You can download the official NXT "Fantom" driver from the downloads page of the Mindstorms website - just scroll down until you see the "NXT Software Download" section. Alternatively, you could just download and install the full NXT 2.0 software package from that same source - that's probably your best bet to get things working.
EV3 Classroom is supposed to work with the NXT brick.