Your suposition is that the power of the purse still applies here. Trump has operated the executive as a collective slush fund, many times using horrible arguments that fail in court and then proceeding anyways. Also he’s allocated money from military coup projects like Venezuala to funds not under congressional control illegally.

Bernies plan is to first expand Medicare to all ages. Going off Trump’s logic this would be done by directing CMS by executive order to adjust the rates, the treasury department to allocate the necessary trust fund expenditures, the IRS to apply the necessary taxes, and the justice department to aggressively target every single possible violation in the private health insurance industry. Bernie’s actual plan is to use the bully pulpit as intended to pressure congress to act in accordance with those changes ahead of time, completely different. That’s because Trump moves to erode government into the pockets of the rich and Bernie seeks to build it stronger to benefit everyone. Executive overreach expansion has only been applied to benefit the rich effectively where everywhere else it had failed.