"how on earth does this meddling happen? The executive branch does not have authority to stop expenditures???"
The Justices are charged with following the Constitution. But, practically, the government has expanded well beyond the 18th century Constitution -- even allowing for the Amendments.
The argument I read was that the money was already owed on a CONTRACTUAL basis. The President CAN'T have authority to not pay bills which the government is already legally/contractually obligated to pay. Or, if the President does have that sort of "ex post facto" then we are all screwed beyond fixing.
So I think the "Conservative" Justices saying "the contracted bills need to be paid" has some common sense behind it. At the same time, I think Berret has more than a bit of "woke" "liberalism" in her as well, and she wasn't the best choice Trump ever made...and now we need to live with her for a long time.