connedting studio display to 2020 27" iMac. Which cable?

In this case, the cable "shipped in the box" with that Apple display is an Apple super-engineered cable.

For most manufacturers, the cables 'shipped in the box' are open for substitution at the factory, and the ones actually shipped are often "lowest bidder" cables. These can be barely adequate to get a picture, and may not support the highest resolutions.

The 27" Apple Studio Display (the 5K one) ships with a 1-meter Thunderbolt cable in the box. Unless there is some compelling reason to get another cable, why not use that cable?

this:

Apple Studio Display 27-Inch (Retina 5K)

That high a resolution requires a ThunderBolt-3 cable no longer than 0.8 meters for Apple super-engineered cable, or no longer than 0.5 meters for brand-X cables, or an active Thunderbolt cable (much more expensive).

TB3 cable from the TB port of the Studio display to one of the TB ports on the iMac. should work.

which studio Display:

Apple Studio Display (LCD)

15.0" (15.1")

Apple Studio Display (Blueberry) (LCD)

15.0" (15.1")

Apple 17" Studio Display (CRT)

17.0" (16.0")

Apple 21" Studio Display (CRT)

21.0" (19.8")

Apple Studio Display (Graphite) (LCD)

15.0" (15.1")

Apple 17" Studio Display (Graphite-CRT)

17.0" (16.0")

Apple 21" Studio Display (Graphite-CRT)

21.0" (19.8")

Apple 17" Studio Display CRT (ADC)

17.0" (16.0")

Apple 15" Studio Display (LCD/ADC)

15.0" (15.0")

Apple 17" Studio Display (LCD/ADC)

17.0" (17.0")

???