Most monitoring projects start with simple questions
What changed, when did it change, how bad is it, who should know, and what should happen next? Temperature, humidity, vibration, leak, power, door, motion, air quality, and machine-state sensors become useful when they produce history, alerts, and operator-readable context.
The architecture is usually sensor, gateway, storage, dashboard
ESP32-class nodes can publish telemetry with MQTT or HTTP. A Raspberry Pi or embedded Linux gateway can buffer locally, normalize data, and forward to a database or dashboard. Alerts can go to email, SMS, Telegram, Slack, or a human review queue.
Accuracy and maintenance matter
Cheap sensors may be fine for a pilot, but serious monitoring needs calibration notes, power planning, heartbeat checks, offline behavior, and a clear path for replacing failed devices.