Yes - its all about habituation - boring dog trainer nerd thing coming
We can't actually actively train in the way most people understand training, ie operant conditioning - 'you do x = you get a treat', for anxiety related stuff, because we're seeking to alter the underlying emotion.. and because we'd be building anticipation of the reinforcer... which slips into frustration bloody quickly.
Most Seperation anxiety sufferers are also suffering from frustration, so adding more by trying to use food or toy reinforcers just makes that worse.
So we're left with desensitization and habituation to build myriad positive experiences, below the dogs threshold for stress/anxiety, and doing that effectively means picking your moments very carefully when your dog is not anticipating/expecting something fun (typically walk, training, play or food).
It also means starting when they are already calm and thats where many go wrong, trying to start an absence immediately after taking the dog for a high octane knackering walk, thinking 'dog will be fulfilled and want to sleep'... And not realising that the stress hormones produced by such a walk (unless it involves a loooooooong slowwwwwwww sniffy walk home, which it typically doesn't) are still going UP for around half an hour after the experience ended... ie, after they get home.
So they head out and the dog is still high on adrenaline and cortisol and has no where for that to go and no one to guide them... and it tends to be a disaster ridden experience, but they think the dog can't have been tired enough so they do MORE next time... and on and on it goes!
So its a fine balance to find, with a lot of factors to consider!
And yes, when we get it right, eventually the visual cues (shit you do to get ready) of your absence routine become the trigger for 'and now lets settle down and have a kip' from your dog!