I don't know what a 'boot drive' is.
A "boot drive", also known as a "startup disk", is the drive that contains macOS, the Mac operating system.
I want to know if I unplug the backup drive (TM) will the files remain on the backup drive when I delete files from the iMac in order to create more space on the iMac. I am no longer interested in backing up any more files on this iMac. All the files should already be on the backup drive.
You should not rely on Time Machine as a way to archive or store files, that's not its purpose. Time Machine backs up the contents of your computer, and it will delete old backups (and the files no longer on your computer) as needed.
What may work best for you is to either store large files on a separate external hard disk drive, or if your "older" iMac is new enough, you may be able to store all your files in iCloud and use iCloud optimization, which it sounds like you may already be doing for some files?
To proceed with the best advice possible, can you please let us know what is the exact year of your older iMac? You can find this information from the Apple menu (top left) > About This Mac.
Jack