There is a great book on dog training called How to Behave So Your Dog Behaves by Sophia Yin. The basic premise is that all animals, including humans, are "trained" 100% of the time. That is to say, there is no explicit training time vs. non-training time. Each time an animal does anything, they either get a positive or negative reward. E.g. if a dog barks and you pay attention to it, then the dog learns to bark for your attention.
A corollary to this is that it's very hard to get a dog to unlearn something once it has learned it, especially if you can't remove the positive reward. For many dogs, behaviors like barking, chewing, and digging are self-soothing. That is to say the dog feels rewarded just for performing the actions when stressed. For this reason, battling entrenched separation anxiety is very difficult and takes many months, as the author experienced.
I wish this information was more well known. Dogs, as puppies, are very malleable, and you can raise a well adjusted dog pretty easily provided you're willing to put in a lot of work for the first few months of life. Once dogs reach adulthood, things become much harder as you're having to unlearn entrenched behaviors in addition to teaching new ones. This is especially important to understand if you're considering rescuing a dog. Rescuing adult dogs is noble, but you're basically playing the lottery with potential problematic behavioral issues.