How do I connect a second monitor to my iMac?
How do I connect a second monitor to my iMac (macOS Catalina, Late 2015 iMac, has 2 Thunderbolt ports)
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Use one of the Thunderbolt ports with an adapter for whatever input your monitor has.
Common monitor inputs:
Thunderbolt 2 / Mini Display Port to HDMI: Belkin 4K Mini DisplayPort to HDMI Cable (2 m/6.5 ft.) - Apple
Thunderbolt 2 / Minidisplay Port to DVI: Mini DisplayPort to DVI Adapter - Apple
More details here: Use multiple displays with your Mac - Apple Support