Lenovo has teased what could be the rumored Yoga X tablet on its Weibo account with a very interesting new capability. The new tablet is pictured hooked up to a Nintendo Switch over an HDMI port allowing it to do double duty as a portable monitor.
https://www.notebookcheck.net/Lenovo-Yoga-X-tablet-teased-doubles-as-a-portable-monitor-using-a-built-in-HDMI-port.537658.0.html
This feature should be implemented on all higher end tablets and was looking for one to do this exact thing for a long time.
Hope it comes out soon
There are software that provides that, one is called duet. I have used that with my iPad. Not as good as a real external monitor, but it works and does the job. But a little slow and fiddly to setup.