Linux – how to access homedir by root user

It sounds like you want to change the HOME dir, not the root dir. Root will always be / or /root.

usermod -d /path/to/new/homedir/ username

Scp works by making an ssh connection to the remote server, then using that connection to execute another scp command on the remote system. The local scp instance and and the remote instance communicate through the ssh link to send or receive files.

OpenSSH scp in particular constructs an scp command string to be run on the remote system. Then it launches ssh to run that command. The eventual ssh command invoked is the equivalent of this:

/path/to/ssh [ssh options...] "scp [scp options...]"

• /path/to/ssh is normally a built-in path to the ssh program.
• [ssh options...] is one or more options to the ssh program.
• "scp [scp options...]" is a single argument containing the scp command and its arguments to run on the remote system.

OpenSSH scp doesn't provide much in the way of options to alter the form of the remote scp command. There's no way to have it invoke something other than "scp", or to insert something like "sudo" in front of the "scp" part.

As you've noticed, scp does have an option to invoke some other program rather than ssh. Someone who knew how could write an ssh wrapper program and have scp invoke the wrapper instead of invoking ssh directly. The wrapper would have to examine its command-line arguments to find the one containing the scp command, alter the command as desired, and then invoke ssh with the altered command-line arguments.