I would recommend Ruuvi Air, its sensors are high quality, and is quite open for devs to do pretty much whatever with it. Works by broadcasting sensor data over BLE, so if you have a smartphone or a home assistant server with BT connectivity, you can display or store the data live. The iOS app sends alerts when different custom thresholds are crossed, like for co2 ppm.

They do have a gateway product, but it’s not necessary if you have HA. If Apple homekit routers supported BLE as source it would work seamlessly in the ecosystem, but a bridge software is required on HA.