Can I use an SD card with the new iMac 2021 24inch? I'm freakin' out!

This is one option that is compatible with the iMac 24 inch 2021: https://www.apple.com/shop/product/MUFG2AM/A/usb-c-to-sd-card-reader

Plenty on Amazon. here is an example:

https://www.amazon.com/Aluminum-Supports-SanDisk-Professional-CompactFlash/dp/B06XSSHZ63/ref=sr_1_14?dchild=1&gclid=EAIaIQobChMI1J__9Kvj8gIVSXFvBB3O8gXXEAAYASAAEgJLZfD_BwE&hvadid=153740853795&hvdev=c&hvlocphy=9016894&hvnetw=g&hvqmt=e&hvrand=5363202355720354865&hvtargid=kwd-147489575323&hydadcr=18031_9433042&keywords=usb-c+sd+card+reader&qid=1630690618&sr=8-14

You need a USB 3.0 (small USB C plug see below) SD card reader. There are many to chose from at not too much money but avoid the larger USB plug unless you have an adapter.

Amazon.com has a number of USB C to SD Card (and other card types). And most are less than ½ of Apple's reader.

Yes, using an external USB-C card reader or a USB-C Dock that has a card reader.

Yes, there are multiple Docks available that have card readers on them. For example:

https://eshop.macsales.com/shop/docks/owc-thunderbolt-3-dock

Yes

You need USB-C to SD Card Reader. Available from Apple Shop £39.00 in UK