Imac late 2015 erase and restore

Backup either using Time Machine or other app Like Carbon Copy Cloner.. Make sure the backup is good by accessing it.

Boot to Recovery and erase the startup drive using Disk Utility that is in Recovery, and reinstall macOS via Recovery

How to reinstall macOS - Apple Support

I would only restore/migrate users and not apps or settings in order to get a clean install. You can apple you setting and redownload and install apps that do not come with the macOS.

To restore from Time machine backup see:

Restore your Mac from a backup - Apple Support

I would only restore your users and not app or settings. The uses include all you data like calendar, photos and documents.

It appears you have a Fusion Disk, that is a small (yours is 24GB) SSD with a larger HDD, ures is likely 1TB.

Yes you want to erase both the SSD and HDD. You may have to recreate the Fusion drive using these instructions:

How to fix a split Fusion Drive - Apple Support