connecting new 27" iMac 5k to 2 additional monitors

Since you have identified the displays, here is what is included with each monitor, and which would preclude your purchasing extra cables:

Apparently, these two displays can be daisy-chained with Thunderbolt 3. There is a product manual link on that Best Buy link you provided that may better illustrator connectivity.

If those additional 4K monitors have DisplayPort, or HDMI 2.0 capable inputs, then you will need an appropriate USB-C to DisplayPort cable, or a USB-C to HDMI 2.0 cable — for each 4K display. The USB-C cables plug into your two Thunderbolt 3 ports on the 2019 iMac (as no new iMacs have been released in 2020).

Yes, have a look at > https://support.apple.com/guide/mac-help/use-multiple-displays-mchl7c7ebe08/mac

You a Thunderbolt 3 to Thunderbolt 3 cable for each monitor.

Those Displays are Plug n’ Play.

If you look in the Overview section of the link for those displays, you will see that they come with a Thunderbolt 3 / (USB-C) cable.

You’re welcome.