can i use iMac (Retina 5k, 27-inch, Late 2015) as target monitor?
2014 iMacs were the last to support target display mode as these were before the 5K screens were introduced, which requires too much bandwidth.


27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable […]
All iMac models introduced in 2011, 2012, 2013 and mid 2014: Use a Thunderbolt cable […]


Furthermore, your 2020 iMac is too new to support target display mode even if you had a ≤2014 iMac to use as a display.

You can connect any of these older iMac models to another Mac from 2019 or earlier

Use your iMac as a display with target display mode - Apple Support