How can I tell what ports my iMac has?

iMac (24-inch, M1, 2021) - Technical Specifications

Connections and Expansion

• Two Thunderbolt / USB 4 ports with support for:   
  • DisplayPort
  • Thunderbolt 3 (up to 40Gb/s)
  • USB 4 (up to 40Gb/s)
  • USB 3.1 Gen 2 (up to 10Gb/s)
  • Thunderbolt 2, HDMI, DVI, and VGA supported using adapters (sold separately)
• 3.5 mm headphone jack
• Gigabit Ethernet
• Two USB 3 ports (up to 10Gb/s)

Take a tour: iMac - Apple Support

This photo is from iMac - Technical Specifications - Apple

The difference is the example of the left is the entry-level iMac. Port identifiers by iMac version are clearer in the original document. Suggest you look at it.

Yes. You can use a USB-C adapter to connect a SD card reader to the iMac.