DIY - Use your Smartphone as a DSLR Monitor - YouTube
When the camera is connected to an external device via HDMI during movie recording, no image appears on the camera monitor. | Sony UK
$50 HDMI Camera Monitor - YouTube
Godox GM55 5.5 Inch IPS Touch Screen On-Camera Monitor, 1920x1080 Camera Video Monitor, 4K HDMI DSLR Field Monitor, Support Focus Assist with 3D Luts, External Camera Monitor for Sony Canon Nikon Fuji:
Feelworld T7 Plus 7" 3D LUT External Camera Screen - Feelworld UK – Feelworld UK Limited
Why would I want an external recorder/monitor?: Digital Photography Review