The only two systems that I know of that have integrated both WiFi Mesh and Zigbee radios into their devices are the SmartThings WiFi Mesh system, and the Amazon Eero 6 system.
The SmartThings WiFi system is not something I would recommend to anyone. You'll get a router that relies on cloud connections, and a SmartThings hub that relies on cloud servers for just about everything. Neither half of this solution is very well regarded these days, and is unlikely to be well supported by Samsung as they have stopped manufacturing SmartThings hardware, instead choosing to allow 3rd parties like Aeotec to do so. This solution would be a complete replacement for Hubitat, as the Zigbee radios in the WiFi mesh nodes cannot be used as Zigbee extenders for other systems. All automations would need to be accomplished with the SmartThings system.
The Amazon Eero 6 and Eero 6 Pro solutions are much better received by users. Each Eero 6 node can handle up to 50 Zigbee devices, paired to that specific node. If a node goes down, those devices are no longer available to Amazon Alexa, until they are paired with another Eero 6 device. See https://support.eero.com/hc/en-us/articles/360051885311-What-is-Zigbee-Smart-Home-Hub- for more details. This solution would be a complete replacement for Hubitat, as the Zigbee radios in the WiFi mesh nodes cannot be used as Zigbee extenders for other systems. All automations would need to be accomplished with the Amazon Alexa system.
Thus, if Hubitat is your home automation platform of choice, then I would look at using either multiple Hubitat Hubs OR the small USB Zigbee repeaters that @aaiyar has mentioned. However, if you plan on installing mains powered Zigbee devices in each of these buildings, then those devices will also be Zigbee repeaters and may eliminate the need for dedicated USB repeaters. Also, please make sure that the Mesh WiFi solution you choose allows the selection of the 2.4GHz WiFi channel used. This will allow you to ensure that there is no significant overlap with the Zigbee 2.4GHz channel. Some Mesh WiFi systems make this decision for you, unfortunately.
