If you want to use a screen as a portable monitor, it must natively accept at least one standard video output. Composite, yPbPr, VGA, and HDMI are the common ones. You can't just use a tablet device as a monitor
If you have an iPad gen 1-4 you can use use a kit like this https://www.aliexpress.com/item/4000539812281.html to use its screen as a monitor. (that kit is only for gen 2 so find a different one for a different gen)