I am confused. I use SFTP to connect to my remote host. I set the SFTP sever to /usr/lib/openssh/sftp-server

What is the next step?

after that I logged in to the remote host. Still I can not view the root folder (/root) or edit /etc/apt/sources.list

Please read FAQ:
https://winscp.net/eng/docs/faq_su

Hi,
I have asked a question about saving edited files that need sudo password when we use internal editor (https://winscp.net/forum/viewtopic.php?t=8919)

It seems that current version does not support that. So I thought it may be a good idea to request such feature.

As you know, I can open the file (for example .bashrc) with the internal editor and after edit, save it directly.

But what about if I want to edit /etc/apt/sources.list or other text files that need root password?

Please consider that for next releases.