$ doas rcctl enable xenodm
$ doas rcctl start xenodm
$ doas pkg_add tigervnc
$ vncpasswd
$ vncserver

You want to use a vnc viewer like ssvnc to connect

WARNING: This setup has zero security. Everything you type including passwords can be keylogged.

How to set up an SSH tunnel for VNC
How to set up an SSL tunnel for VNC (incomplete)