My 10th grade class is only taking me so far

So the legislative branch passes laws and allocates funding to those laws. The executive branch puts those laws into action.
Can the executive simply just say, nah?

Because isn't that what Trump and them are doing (re: the pause in federal funding to ensure it aligns with the president's priorities, shutting down USAID)?

Is this a unique thing for this administration, or something that is always done? If it is what is always done, it makes me question the point of Congress's existence.

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