Zigbee devices can be used in a closed mesh network, without an internet connection or any proprietary cloud or subscription. One needs a USB coordinator dongle [1], a small computer like raspberry pi, zigbee2mqtt [2] & Home Assistant [3]. It works perfectly. Devices implement various standardized "profiles" documented in an open standard ZCL/dotdot and a lot of devices from different manufacturers work right out of the box.

[1] https://sonoff.tech/product/diy-smart-switch/sonoff-dongle-p... [2] https://www.zigbee2mqtt.io/ [3] https://www.home-assistant.io/