I have a 2018 Mac mini which has the same USB and Thunderbolt 3 capabilities as your Mac. I also have a Thunderbolt 3 Samsung X5 SSD and a 10 Gb/s USB Samsung T7 SSD. I will base my answers on what I know about this hardware.
When the Thunderbolt 3 Samsung X5 is connected to the Mac through a USB Type-C receptacle (jack), System Information displays the following under NVMExpress.
Like the internal drive, the Thunderbolt 3 Samsung X5 appears as a NVMExpress drive, except the drive is external. Note that TRIM is supported.
System Information also displays the following under Thunderbolt.
Since the speed shows up to 40 Gb/s, one can conclude the read speed of 22.4 Gb/s is not limited by Thunderbolt 3, but rather by the NVMe drive inside the Samsung X5.
When the USB Samsung T7 is connected to the Mac through a USB Type-C receptacle, System Information displays the following under USB.
The data transfer speed is unto 10 Gb/s. This is the maximum speed for USB devices connected a USB Type-C receptacle on both of our Macs. There is no indication of TRIM support, which may not be necessary if the embedded PCIe NVMe technology is sufficiently fast.
When the USB Samsung T7 is plugged in a USB Type-A receptacle on the Mac, System Information displays the following under USB.
Note: The USB Samsung T7 comes a cable for plugging into a USB Type-C receptacle and a different cable for plugging into a USB Type-A receptacle.
The data transfer speed is unto 5 Gb/s. This is the maximum speed for USB devices connected a USB Type-A receptacle on both of our Macs. Again, there is no indication of TRIM support.
Conclusions
You post contains the following four questions.
What is the fastest external storage supported on Intel 2020 27" iMac?
The Thunderbolt 3 Samsung X5 has a read speed of 2800 MB/s. In terms of bits per second, this would be 22.4 Gb/s, since 8 bits = 8 byte. This means the data rate between the external drive and the Mac must exceed 22.4 Gb/s, which is faster than the maximum USB data rate of 10 Gb/s offered my your Mac. From this, the following conclusion can be made.
>Nothing that USB offers will be faster than what Thunderbolt 3 offers and also be compatible with your Mac.
What does the 2020 Intel iMac 27" support?
The The USB Type-C receptacles support at least the following devices.
Self powered Thunderbolt 1
Self powered Thunderbolt 2
Thunderbolt 3
USB 2.0
USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)
USB 3.1 Gen 2 (same as USB 3.2 Gen 2×1)Note: Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter does not provide power from the Mac to the device.
The The USB Type-A receptacles support at least the following devices.
USB 1.1
USB 2.0
USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)What does that mean for data transfer speeds?
The USB Type-C receptacles offer Thunderbolt 3 speeds up 40 Gb/s and USB speeds up to 10 Gb/s.
The USB Type-A receptacles offer USB speeds up to 5 Gb/s.
When buying an external drive what do I need to look out for to make sure it will be compatible with the fastest option?
Look for a Thunderbolt 3 SSD with high with read/write speeds (in the GB/s range). The SSD should also support TRIM. You can also look for a Thunderbolt 1 and 2 SSD, but these drives would need to be self powered and would require a Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. However, finding a self power Thunderbolt 1 or Thunderbolt 2 SDD at a price lower than Thunderbolt 3 SSD is unlikely.