How can I use USB-C ports for charging my iMac?

iMac (24-inch, 2023, Four ports) - Tech Specs - Apple Support

On the four-port version of the 24" M3 iMac, the two USB-C ports marked with a lightning bolt symbol are multi-purpose ports with support for

• USB 3.1 Gen 2 (up to 10 Gb/s)
• USB4 (up to 40 Gb/s)
• DisplayPort
• Thunderbolt 3 (up to 40 Gb/s) (hence the lightning bolt symbol)

They adapt to whatever is plugged into them. There is a limit of one external display, but you can use either port to connect it.

The two USB-C ports without the marking are single-purpose ports with support for

• USB 3 (up to 10 Gb/s) – another way of saying USB 3.1 Gen 2

The two with lighting symbols are Thunderbolt 3/USB-C ports the other two are plain USB-C ports.

The Thunderbolt ports can be used for either Thunderbolt and/or USB devices.

The plain USB-C ports can only be used for USB devices.

You can plug your previous USB-A devices into any of the ports using USB-C to USB-A adapters.

for some examples:

USB-C to USB Adapter - Apple

https://www.amazon.com/dp/B07PDLQ555/ref=sspa_dk_detail_0?

https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275

The iMac can not be charged, it must use the supplied power adapter cable.

You don't charge an iMac. Those are Thunderbolt symbols for Thunderbolt devices.