To recover data from the HDD portion of the Fusion Drive, you'll need the original SSD and a professional data recovery tool like iBoysoft Data Recovery for Mac. It can retrieve files on the Fusion Drive and allow you to preview them for free.
Here's how to recover dat:
Step 1: Remove the SSD portion of the Fusion drive from the failed Mac.
Step 2: Plug it into an external enclosure and connect it to your new iMac, along with the HDD.
Step 3: Download and install iBoysoft Data Recovery for Mac.
Step 4: When the app is open, select "Fusion Drive" from the left pane.
Step 5: Select the SSD and HDD of your Fusion Drive and click "Search for Lost Data."
Step 6: Preview your data.
Step 7: Select the files you want to recover, click the Recover button, and choose a location to save them.