How can I access the Linux desktop?

[Dark Corner]

I have a Xubuntu PC, latest LTS, on which a RealVNC session is already
active.
I think I understand that I can't use RVNC with Guacamole, so, how can I
access the PC desktop?
Access must be complete, for example also with File Transfer, and must be
shared to assist the user who must therefore see what I am doing.
Thanks in advance.