So, your 2020 iMac shipped was shipped with macOS 10.15 Catalina, so it never had the iTunes app installed. It's always been the Music app there. Any Mac you had before that would have had iTunes on it, and depending on the actions you took when you moved to your newer Mac, your music files may or may not have been properly transferred.
If you are confident that all your music files had been transferred to this iMac and you've been using some form of backup, you definitely should explore recovering those files from the backup.
Change where your music files are stored on Mac - Apple Support
Obviously, your music files should not have been affected by any macOS update or upgrade, but sometimes things happen. If you had run into some problem and had to deviate from any normal upgrade process, then it's certainly possible that some files were lost.
Keep your 🤞🏽 and hope for the best.