How do I connect USB devices to a new iMac with Thunderbolt ports?

The Thunderbolt ports also support USB. You may need some USB-A to USB-C adapters like this in order to plug in your USB devices that have the older USB-A type connector.

More specifically, on the new 24" iMac models, the Thunderbolt 4 ports support the following:

• Thunderbolt 4 (up to 40Gb/s)
• USB 4 (up to 40Gb/s)
• USB 3.1 Gen 2 (up to 10Gb/s)
• DisplayPort

Note also that since it supports USB 3.1 it is also backward compatible with earlier versions of USB.

To, hopefully, clear things up a bit, when discussing ports on your Mac, there are three distinct things to take into consideration:

1. The physical shape of the port. This is typically identified with a letter. That is, -A, -B, or -C.
2. The protocol standard/characteristics (data transfer speed, power handling, etc.) that the port supports, regardless of its shape. This is typically identified as a number: 1.0 - 4.0.
3. The USB protocol standards can be supported by all of the USB port types. However, the Thunderbolt 1.0 & 2.0 standards are only supported with another port type, called the DisplayPort (or Apple's version, called the miniDisplayPort). On the other hand, the Thunderbolt 3.0 & 4.0 standards are only supported on USB-C type ports.

In the recent past, most computers and peripherals used USB-A type ports, Currently, world-wide, they are being replaced with USB-C type ports ... so, if you have peripherals that came with USB-A ports, like MartinR has suggested, you will need a USB-A to USB-C adapter to connect them. As I'm sure you are aware, all of the ports on your new iMac are of the USB-C type.

So, what it comes down to is four things:

1. What physical connectors does your peripherals have?
2. What protocol standards does your peripherals support?
3. What physical ports does your Mac have, and
4. What standards do they support. Again, here MartinR has told you that your iMac's USB-C ports can support multiple standards, both USB and Thunderbolt. The thing to keep in mind is that a peripheral that only supports, let's say the USB 1.0 standard, will not be "upgraded" to support the USB 4.0 standard, even if your iMac can support it.