I don't have the HE yet so am lacking knowledge. I understand how the HE connects to zigbee/zwave devices but how does it connect via WiFi? For example, the sonoff wifi wall switches.
Via specific drivers (if they exist) and most usually fixed ip addresses for your devices.
Hubitat has a LAN connection, which is how you connect to the web interface for setup and administration. Wi-Fi devices are (in the way most consumer networks are set up) accessible from this same LAN, so Hubitat--like any device on your network--is able to connect to them over standard TCP or UDP protocols (as long as they are accessible from your network).
The real issue is whether there is a documented or reverse engineered API for such devices. The Hue Bridge is technically wired (not wireless) but is a great example of a IP-connected device with a great, documented, local interface that Hubitat supports natively. Sonoff doesn't have an official Hubitat integration, but there is a community driver that works for many. The important thing is to not assume that Hubitat can connect to any random Wi-Fi IoT device--most are cloud-only, have only undocumented or at best reverse-engineered interfaces, and work poorly if at all outside their manufacturer's "ecosystem."
And as noted above, most of these work a lot better (as will your hub) with a "static" IP address. Some may let you set one directly, but I normally prefer to do it via DCHP reservation on my router by MAC.
Thanks for the detailed reply.
I understand the network connectivity but was wondering how the devices actually communicated (e.g through relevant device app or direct) which you've answered now, thanks.
I'm assuming, for any devices without drivers, hubitat can communicate via IFTTT if supported (although losing local control). Is that correct?
Yes, there is an IFTTT integration with Hubitat so pending a direct API integration for your devices, you can use that. Of course the normal perils of Clihd, slow speed, missed transactions may arise
I use it for several Smart Life and eWeLink based devices and it works OK with around a 2-5 second delay being typical.