How to connect an external HDD that has a USB cable to an iMac 24 inch , M1, 2021?

Welcome to the Apple Support Communities.

You can use individual Apple adapters: Adapters - Power & Cables - Mac Accessories - Apple. You could also use a dock solution, such as a CalDigit TS3 Plus Dock - Apple or look at OWC Docks - MacSales.com (OWC).

Cheers,

Jack

iMac 24” M1 is available in a two-port configuration with optional Ethernet, and a four-port with Ethernet configuration.

Given the choice, go for 16 GB memory for best useful longevity, and as much storage as you’ll need over the lifetime of the iMac. There are no memory upgrades, and no internal storage upgrades. Ethernet can be added to the low-end configuration.

USB-C is a newer connector and supports various connections including (in this case) both Thunderbolt and USB connections, and dongles and docks are available to connect from Thunderbolt to a plethora of other connections, including to the older and familiar USB-A connection, and docks some with SD support wouldn’t surprise. There are also USB-C USB connections and dongles for USB-A devices. USB-A adapters for flash device support are available, too.

Some of the many choices:

https://eshop.macsales.com/shop/owc-thunderbolt-dock

https://www.brydge.com/pages/docking-stations

https://www.apple.com/shop/product/MJ1M2AM/A/usb-c-to-usb-adapter

I’d usually suggest acquiring a printer and scanner with wireless support, or with a wired connection. Preferably with AirPrint and/or IPP/IPPS printing, and TWAIN support for scanning. This AirPrint path particularly if there’s any likelihood you’ll be printing from another computer or from an iPad or iPhone.

Some details on the printer choices here: Which Printer Should I Buy? - Apple Community

I’d suggest a USB-C hub or dock with an external power supply. Almost as good as a thunderbolt dock, but much cheaper.

Just make sure to pick one with its own power supply: bus powered options work for connecting printers, SD cards, wired mice, and other low-power or self-powered items but would struggle with multiple hard drives and iPhone charging.