All your Zigbee devices are stored in the Z2M/ZHA backup.
When you update the Zigbee firmware, each time it updates, it erases the Zigbee chip's memory, but your devices remain because Z2M/ZHA restores (writes to the coordinator's memory) them when the Zigbee network starts.
If your devices disappear, you should contact zigbee2mqtt/ZHA support to resolve this issue.
If you still can't find an answer to what you're looking for, or you have a specific question, open a new ticket and we'd be happy to help!
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