That was terrible advice instructing someone to install two versions of macOS to the same drive as that usually leads to problems including getting confused where your data is stored. If you need access to an older or newer version of macOS or just want to try out the newer version before upgrading your main drive, then installing an OS to an external hard drive is a much safer option as it is less likely to impact your main macOS boot drive.
The simplest method moving forward is probably to boot into the OS you want to remove. Manually transfer your files from within your home user folder(s) to an external drive which is partitioned and formatted as GUID partition and MacOS Extended (Journaled). If you have multiple user accounts on that OS, then make sure you transfer the data from all those user accounts.
Once you have transferred all the important files from all the user accounts on the OS you want to delete, then you reboot the Mac into the other OS which you want to keep. Now you will need to delete the APFS volumes and/or APFS Container associated with the other OS. In theory macOS should not allow you to delete any APFS volumes or Containers for the OS you are booted into.
Another more drastic option (perhaps simpler option) is to perform a clean install of macOS by erasing the entire physical drive before reinstalling macOS. You can then restore or migrate from a TM backup as long as the OS for the TM backup is for the same or older OS. Again make sure to have a good backup of both operating systems or make sure you have transferred all your important files to an external drive as previously mentioned.
Make sure to disconnect your external drives and backup media before attempting to delete any APFS volumes/Containers, or before erasing any drives. You don't want to accidentally delete or damage your backups or externally stored data.
After you get back to a single OS you will need to figure out which files on the external drive from the now deleted OS are unique and which of those files may be newer versions of the ones on your current user account. There is no easy way to avoid this step. It will require you to look at each file.
In the future if you ever use more than one OS on a computer or if you have multiple computers, then make sure to store your files in a shared location so that you have just a single location where you make modifications to those files. Having them stored on an external drive or a network share are two options instead of storing them within the home user folder of any one system. A cloud service can allow you to sync files between multiple devices, but some of them may affect system performance plus you risk someone gaining access to anything stored in the cloud. No matter how you do this you will want to make sure to have a good backup of your computer and any external media (includes external drives, network drives/shares, or items stored/synced in the cloud).
I find it is best to keeps things as simple as possible as it makes management much easier and it is less likely for mistakes to happen.
No matter what you should make sure to have a good backup of both operating systems just in case you make a mistake.
You should always have frequent and regular backups.
