Please follow these steps:
First, you need to pair your Bluetooth device with your smartphone. I am using the realme band to explain.
➡ Go to Settings
➡ Select Bluetooth
➡ Click on the available Bluetooth device and pair it.
Now
➡ Go to Security ➡ Smart Lock ➡ Select Trusted Devices
➡ Click on the Add Trusted Device and select a paired device.
➡ Click on the Yes, Add and you do it.
Note: If you do not find your device, turn Bluetooth off and Turn it on back.
You can use it by following these steps with all your Bluetooth devices.