Why does certain device can be detected in bluetooth and some cannot?

We tried opening the bluetooth adapter in pi4, some devices where able to be detected.
Like oneplus 6 phone when bluetooth is opened, we can see the name of the phone in Developer Tools > States. The phone is not shown in integrations
However, we have another random company’s smartplug with bluetooth connection. That smartplug cannot be detected by HA.
The smartplug can be detected with phone by bluetooth normally, so bluetooth of smartplug works.
Although the phone can be detect, we cannot seem to add the phone into HA.
Why does this happen?

Because no one wrote an integration for that smartplug.

OR you use USB3 on your RPI4 as well. That limits the bluetooth range tremendously…
But yeah, most likely “random company” is just not supported by HA.