iMac 17,1 27" as a monitor for a Mac Pro Cylinder (late 2013)

You cannot.

Read carefully the limitations of this deprecated tech.

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

Per the link above:

Only the following older iMac models could be used:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.

• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.
• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

MacTracker says that iMac17,1 is an iMac (Retina 5K, 27-inch, Late 2015).

Target Display Mode went away when the first 27" 5K iMac came out in Late 2014. It never returned, so your iMac doesn't support it. Apple also later added restrictions such that no Mac released after 2019, or that is running any version of macOS later than Catalina, supports using any Mac as a Target Display. That would rule out using your 2013 Mac Pro with an older, non-Retina iMac running in Target Display Mode unless you were willing to downgrade the Mac Pro to Catalina or earlier (not recommended).