If you happened to install Catalina on a disk which already had files on it, this is where you find them after you boot into the new macOS:
System > Volumes > Macintosh HD (or your disk name)
Hope this will save you from otherwise pending heart attack 🙂 In my case, I upgraded the macOS on my personal iMac 5K with 2 TB SSD replacement for what originally was 1 TB Fusion drive.
If you don't know yet, the Fusion drive consists from a smaller capacity SSD "blade" flash storage and a large but cheaper traditional hard drive. I replaced the HD with larger SSD and configured it as two separate volumes.
For over a year, the 128 GB SSD "blade" was my boot drive. I used the installed 2 TB SSD as my "data" volume, for large Photos, Final Cut and iTunes libraries. But now I installed the Catalina on the large drive. It gives me more "breathing space" than having macOS, Applications and user folder on the smaller drive - which I'm going to use as a temporary volume for downloads and as a scratch drive.
In Catalina, the whole System folder is now on a separate, read-only "partition" (technically, it's an APFS volume), which makes your system much safer against malware. That's why the files don't show in the top level of the volume after the upgrade.
If you still have traditional hard drive in your iMac - come to us for an upgrade! Prices of SSDs went down rapidly, we have reasonable offers or you're welcome to bring your own, and being an Apple Authorised Service Provider, you won't lose warranty or AppleCare on your Mac.
SSD is much faster and more reliable, uses less power (generating less heat and extending battery life in portables) and is completely quiet. It is still more expensive per GB but totally worth it. Ask us for a quote and mention how much space your data and apps currently use.
PS: if you're seasoned Mac user enough, you may remember the original "Where_have_all_my_files_gone" 😉 Do you? Mention it in comments!