Why so many builds?

Different CUDA versions support different hardware (compute capabilites), and different driver versions support different CUDA versions. As such, we need multiple builds to support all the various hardware/software combinations.

GPUSieveSize and older GPUs:
Older GPUs (all GTX 1xxx and older, and any with less than 8GB RAM) may have trouble running with the current default GPUSieveSize setting of 2047. If you have problems, set it to 128 in mfaktc.ini and try increasing it (256, 512, 1024). Higher values give slightly better performance at risk of lag (or crash) if the GPU has insufficient RAM.

Use mfaktc with AutoPrimeNet.

Note: CUDA on Mac OS was discontinued years ago. As such, there a certain caveats to running mfaktc on a Mac.

There is no support for CUDA on macOS versions after 10.13 High Sierra.

You will need to install the CUDA driver, and possibly the Nvidia Web Drivers.
CUDA 4.2 has been tested on Mac OS X 10.6 with stock drivers, and CUDA 10.1 on MacOS 10.13 with the latest web drivers.
Anything else you will have to figure out yourself...