Using iPhone screen as a display monitor

Solution 1:

An app called Duet can make your iPhone a display for your Mac.

SideCar for iPad

With Sidecar, you can use your iPad as a display that extends or mirrors your Mac desktop.

https://support.apple.com/en-us/HT210380

Duet

Ex-Apple Engineers turn your iPad into an extra display. Now for Mac & PC.

http://www.duetdisplay.com/us

Air Display

"You have a bunch of different options to turn your iPad into a second display, but we’ve had the most consistent luck with Air Display."

https://avatron.com/applications/air-display/

iDisplay

"iDisplay app turns your iPad, iPad mini or iPhone into secondary screen - too good to be true." - MobileCrunch

http://www.getidisplay.com

I had quite good experiences with yam (yet another monitor).

As of the time of writing this is ipad and mac only. It's cheaper than duet.