what happen on builtin command executed as background process

As you seem to understand, shell built-in functions such as echo are processed by the running shell and therefore do not require a new process to be started in the foreground (normally).

However, if you send them to the background, they need to be controlled by a distinct shell and will consequently start a new shell process with the command in question. This should be fairly easy to observe in bash as sending a process in the background will show the new process id next to the job number such as:

$ echo 'hi' &
[1] 19536
hi

In this case, process 19536 is a new bash instance.

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