It appears that PDJT had nothing to do with the firing.
No, it appears that Sec Navy approached the White House with a corrupt deal outside the chain, and Trump called his direct supervisor and told him to deal with it.
This is a Trump act. And it’s how a CEO would do it in a Trump business.
While it’s possible Esper could be empowered to fire one of the service secretaries without consulting his boss, I’d be surprised if Trump wasn’t at least given a heads-up.