With your iMac, you just need to pair the ConnectClip to the iMac and your hearing aids. As per
https://www.oticon.com/-/media/oticon-us/main/download-center/connectclip/pi/14242----15555-10192-connectclip-quickstart-guide.pdf?la=en&hash=93D308A3F0390BF5932005ACB10CB5B1A359C0F3
All iMacs have Bluetooth, definitely no dongles are requried.
One would wish your statement is true but it is not in my experience. I have an iMac Pro (2017, model iMacPro1,1). With older versions of macOS, my ConnectClip (with current firmware at the time) when used with OPN and OPN S 1 aids (with current firmware) would pair but not operate correctly with the iMac Bluetooth (dropouts, stuttering audio, etc). With a BTD800 (USB-A connector) supplied with my ConnectClip, the ConnectClip worked fine. I didn’t need the BTD with a similar vintage work MacBook Pro. When I was trying to figure out the issue, I was told by an Oticon Tech Support person (called by my provider) that I should use the BTD800 with the iMac and it was more likely that it would be needed with a desktop than with a laptop. One macOS update changed the firmware level of the internal iMac Bluetooth; after that the ConnectClip worked better with the built-in Bluetooth but I still use the BTD 800.
Elizabeth,
There are several models of the BTD800 - some fit the old, original USB connector (rectangular) while others fit into the newer USB-C connector. If you have only USB-C connectors on your iMac and have the original BTD800 you can use an adapter (Apple sells them as do many other vendors). USB-C is not orientation specific unlike the original USB connector.
Once you have the BTD 800 connected to your iMac and paired with your ConnectClip, you may have to select the BTD 800 as the output audio device in Sound Settings to route audio through your ConnectClip - mine shows up as a “Sennheiser BTD 800 USB for Lync”.
Note that the firmware of the BTD800 can be updated (and there have been many updates, some of which are related to Teams) using the EPOS Connect program. EPOS Connect seems to finally work OK with macOS but originally I had to update firmware using Windows.
https://www.eposaudio.com/en/us/enterprise/software/epos-connect4