In the Plasma Desktop System Settings Bluetooth applet, it says "Bluetooth is disabled" and gives me an active button "Enable". Clicking it does nothing. It does not switch to saying "Bluetooth is enabled", nor does it give any kind of error message. Tailing journalctl gives no related output. How to tell what is going wrong? Is anything logged or reported anywhere?

Meanwhile the bluetooth tray icon lets me select Add a new device and will happily scan forever without showing anything. Shouldn't it detect that bluetooth is "disabled" and not do this? Or is judging whether bluetooth is enabled some other way than the setting applet and getting a different answer? (Wondering if this could be a clue to as to what's wrong.)

Note that the systemd bluetooth service is running. Everything from journalctl about bluetooth (around startup time) looks ok, for example

kded5[1833]: bluedevil: Bluetooth operational changed true

(Plasma Desktop 5.23.4, KDE Frameworks 5.89.0, QT 5.15.2, Kernel 5.15.7, openSUSE Tumbleweed)

You probably missing a driver or firmware for the bluetooth module.

How to fix it will be dependent on the OS, and not KDE or Plasma specifically.

But one can discover if there are any hardware issues looking at the kernel, logs, either using KSystemLog, or other log viewer, or on the command line:

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$ dmesg | grep -i blue

Usually, unless you have a separate dongle for Bluetooth, it is part of the wifi card, so you can use lspci and/or lsusb to find what you have. Or if yoiu have inxi installed:

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inxi -Eaz

I have exactly the same problem with kernel 6.0.0 (Gentoo Linux). The dongle works fine with 5.19.10 (and earlier versions.) "inxi -Eaz" shows:

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Bluetooth:
  Device-1: Cambridge Silicon Radio Bluetooth Dongle (HCI mode) type: USB
    driver: btusb v: 0.8 bus-ID: 1-2:6 chip-ID: 0a12:0001 class-ID: e001
  Report: bt-adapter ID: hci0 rfk-id: 1 state: up address: N/A

rfkill shows bluetooth hci0 is unblocked. rmmod and modprobe of btusb doesn't help. dmesg shows

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[  363.816623] usb 1-2: new full-speed USB device number 6 using xhci_hcd
[  364.134480] usb 1-2: New USB device found, idVendor=0a12, idProduct=0001, bcdDevice=88.91
[  364.134994] usb 1-2: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[  364.135498] usb 1-2: Product: BT DONGLE10
[  364.175169] usbcore: registered new interface driver btusb
[  364.178239] Bluetooth: hci0: CSR: Unbranded CSR clone detected; adding workarounds and force-suspending once...
[  364.178835] Bluetooth: hci0: CSR: Couldn't suspend the device for our Barrot 8041a02 receive-issue workaround
[  364.179414] Bluetooth: hci0: HCI Delete Stored Link Key command is advertised, but not supported.
[  364.179999] Bluetooth: hci0: HCI Set Event Filter command not supported.
[  366.496339] Bluetooth: hci0: command 0x0c5a tx timeout
[  366.496344] Bluetooth: hci0: Opcode 0x c5a failed: -110

This is indeed a cheap CSR clone dongle. For me, this seems like a kernel regression, but the OP is running kernel 5.15.7, so maybe something got fixed after that and then broken again with 6.0.0? Can anyone suggest anything to further troubleshoot?
I'm on KDE Plasma 5.25.5, KDE Frameworks 5.96.0, Qt 5.15.5