Using a mid 2010 27" iMac as a second screen for a mid 2015 27" 5k iMac [duplicate]

No. You cannot do this. The 2019 iMac is not supported for Target Display Mode.

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Which iMac Models Support Target Display Mode?

General Rule of Thumb: If it's after 2014, Target Display Mode isn't supported.
27 inch iMacs from 2009 - 2010 support TDM via Thunderbolt or mini DisplayPort (mDP)
All iMacs (21 inch and 27 inch) from 2011 through mid 2014 support TDM via Thunderbolt only.
Retina iMacs from late 2014 and on do not support TDM at all.

Which Mac models can you use as a source?

All Mac models through 2019 that meet the above requirements can use Target Display Mode. Currently, 2020 Macs will not connect (i.e. 2020 MacBook Pro running Catalina), but a 2019 MacBook Pro also running Catalina works fine.

What version of macOS is required?

On the Target iMac, it must be running High Sierra or earlier.
TDM is not supported on any iMac running Mojave or later.
It doesn't matter what version of macOS you're using on your source computer

Can I use my iMac as a Target Display for a PC?

Yes! As long as the target Mac meets the above requirements and the PC has the proper port; Thunderbolt or mDP. The OS on the source computer doesn't matter at all.

Can I use a "bricked" or broken iMac as a Target Display?

No. Your iMac must be a fully functional iMac running High Sierra or later.

Sources & Further Reading:

Use your iMac as a display with target display mode
iMac and connected external displays, all as monitors for MacBook Pro?
Using iMac late 2013 as external monitor to HP EliteBook 820 G3 on Windows 10
How could i use an iMac as secondary monitor for a Windows 7 laptop
Using an iMac as a display for PC laptop in dual mointor configuration