External Hard Drives

What will the drive be used for? If it's for Time Machine one of these at about 2 ½ times the size of your boot drive is an excellent choice: OWC Mercury Elite Pro - an HDD.

It it's for data storage and use one of these SSDs would be the way to go depending on your budget:

OWC Envoy Pro FX Thunderbolt 3 + USB-C Portable NVMe SSD**

OWC Envoy Pro SX Rugged Portable NVMe SSD with Thunderbolt/USB4

OWC Envoy Pro Elektron USB-C portable NVMe SSD

Envoy Pro EX with USB-C or USB 3 

For your model contact OWC's customer support and get their recommendation for the SSD model that would best suit your iMac model, work requirements and budget.  They may have additional models other than those I've shown above.

Most any name brand external drive -hdd or ssd- is compatible with the macOS.

Find one with the proper port connector - USB-C - for your iMac. Adapters are available if necessary.

You just have to make sure you format it for Mac use the first time you plug it in.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Also, do not install or rely on any software that comes preloaded on an external drive. To do so is destined to end in disappointment an/or data loss.