Zwave and WiFi won’t interfere with each other: they are on very different frequencies. It’s one of the reasons that zwave was so popular for fixed position devices like light switches: unlike Zigbee, Bluetooth, and thread, it’s just really unlikely that zwave would ever run into interference from Wi-Fi.
Zigbee is a different story. Zigbee, Bluetooth, thread, and Wi-Fi all broadcast on the same frequency, 2.4 GHz. Of these, Wi-Fi is many times more powerful, so it’s easy for it to drown out the messages of the other three. So, getting them all to work together happily can be tricky, which is why the “channel“ becomes important. The 2.4 GHz frequency is broken up into multiple channels which are different parts of that band, so you can separate the messages somewhat by choosing different channels for them.
See the community FAQ for more details:
FAQ: What WiFi Channel is least likely to interfere with SmartThings?