From the Sony website:
You can connect your Xperia® 1, Xperia 5, Xperia 1 II, Xperia 5 II, Xperia PRO, Xperia 1 III, or Xperia 5 III to an external display or TV which has a DisplayPort™ or HDMI® port.
None of the above devices are supported by SFOS. The problem has two sides. Even if the chipset supports it -which many do- it doesn’t mean that the MFG has wired it so it can be used. And then is the SFOS side of things where the compositor doesn’t support external monitors.
There is another thread somewhere in the forum that covers what you ask for but i cant find it right now.