Can I use my 27” iMac display with a Mac Studio?

Target display mode is the feature you're referring to. But it only works between older macs. Support was dropped for new products in 2014 if I'm not mistaken.

If you want more information about the feature and what products are supported see: Use your iMac as a display with target display mode - Apple Support.

Though not a substitution for the main monitor, depending on your goal you may be able to use your iMac as a receiver for AirPlay. See set up here: Set up your Mac to be an AirPlay receiver - Apple Support

see use here: Use your Mac as a display - Apple Support.

You may also be able to use Univeral control allowing you to use both systems simultaneously. Use a keyboard and mouse or trackpad across devices with Universal Control on Mac - Apple Support.

You will still need a display, but hopefully, this gives you some ideas on how to make the most of your macs.

If these suggestions are not helpful for your particular needs, you can also consider trade-in for your old Mac. Apple Trade In - Apple

You cannot.

+1 Nope, no can do. You need a proper external display with the Mac Studio.