1. Does BC end need installation?

No
1. Only need to install on each sub-server and configure the following in config.yml
# Server identifier, required for group servers (/server lobby) can teleport
server: "lobby"
2. Configure to mysql mode

2. PlaceholderAPI common variable explanations

Enter the following commands to install PAPI extension components (choose as needed)
papi ecloud download player 
papi ecloud download vault
papi ecloud download server
papi ecloud download Multiverse
papi reload

3. Manually added channel but messages not received?

Except for the default channel "default", other newly added channels require granting corresponding permissions to players

playerChat.chat.channel_name to receive messages

Third-party plugin registered chat channels don't require permissions (handled by third-party plugins)

4. Two messages appear after installing the plugin?

If you have CMI installed simultaneously, disable CMI's chat function

Path to disable CMI chat:
plugins/CMI/Settings/Chat.yml

Set the following two to false
Chat.ModifyChatFormat: false
Chat.ClickHoverMessages: false

5. How to configure RGB colors?

Configuration format: &#FF0000 example