! is going to invoke an operating system command. The operating system copy command will be invoked.
We can tell by the ..\ that the target is MS Windows. So this is the DOS copy command.
https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/copy
The DOS copy command allows you to specify multiple source files separated by + and all of those are to be combined in order.
In short, the command is adding a footer on to the end of the original file and storing the result into a particular file. I am not on the significance of the final | as I do not use Windows shell much.
