Bluetooth menu list is not loading onto my iPhone 8 plus

What menu are you expecting? Do you have devices that you have paired to your iPhone? When you say it is trying to download, are you seeing the spinning wheel? If you do not have any devices paired to your iPhone, then that spinning wheel is telling you that your iPhone is in pairing mode. It will only discover devices that you would have in your already paired list, or any that are in discovery/pairing mode as well.

What’s happening instead?

Even if you are not trying to pair a device, when you open Bluetooth, it will show the devices you already have paired and you will also see the spinning circle, as opening Bluetooth is what puts the iPhone into pairing mode. That is expected behavior.

Alright. Have you ever paired with it before? If not, you need to ensure that you have put the JBL speaker into pairing mode. Read the documentation that can with the speaker to do that, and just open Bluetooth on the iPhone. If the speaker is in pairing mode, it should appear on the screen. Follow the steps to pair it.

If you do not put the speaker into pairing mode, and then open Bluetooth on the iPhone, the two devices cannot discover each other. See this for assistance. If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch - Apple Support