Heeling and loose leash walking are two very different things. As long as the owner struggles to understand the difference the dog will never be clear with what is expected while out on walks.
Loose leash walking doesn’t have to be on one side as long as they keep slack in the leash. I prefer not to use special tools and use mostly flat collars or martingales, but this can also be achived in a harness. Instead I use a clear system of communication. The idea of loose leash walking is that when the dog feels any pressure on the collar that the pressure is actually a cue that means move closer to my owner and check in. Unfortunately many dog owners accidentally teach dogs to ignore leash pressure and actually unintentionally teach their dogs to pull.
Loose leash walking is communicating with your dog through the leash. I teach my dogs to understand how to give to collar pressure, so that they can easily be moved around by collar pressure. While giving into the pressure of the leash they cannot be pulling because those two behaviors are incompatible.
Heel is a consistent placement of your dog’s shoulder to your leg. If I move backwards, pivot, sideways or any other direction my dog’s position should not change it should be dog’s shoulder to my leg. This position can NOT be taught with a magic collar or leash. It can only be taught through clear and consistent training.