Then you must have skipped 3rd grade as the following EO's for example are (or were) law and do not pertain exclusively to the Executive Branch of the Federal Government.
Executive Order 12566 (Safety belt use requirements for Federal employees).
Executive Order 12561 (Delegating certain functions of the President relating to Federal civilian employee and contractor travel expenses).
Lastly and this is an easy one, Executive Order 12564 (Drug-Free Federal Workplace)
As to the Treasury and 'impounded' funds.
For one, if the 'sanctuary zones' like NYC, Chicago and LA were declared illegal - which they are in fact to begin with but more importantly enforced by the Federal Government, withholding funds would not be illegal. Countless 'strings' are attached to federal funding to cities and programs. Issuance or withholding funding as a condition of compliance with Federal Law, regulation and or statute pertaining to said funding is a 'no brainer' (that is, unless one skipped 3rd grade).
BTW, you might want to read about an EO issued by RR that redefined 'Federalism' and how Clinton was going to 'slightly' revise it. That is until Congress found out the contents and went into an uproar. BJ's EO would have given him unheard powers as POTUS. For instance he could have declared Marshal Law at the snap of a finger. Thankfully do to Congress he never signed it.
Yes, strings are attached to funding. This is done by Congress and the prez has no authority to attach strings. In fact, if you will look at how the congressional oversight committees work, you'll see that Congress can get very detailed in manageing the president's employees by being very detailed in how they appropriate the money.
As for disputes between the the executive branch and the legislative branch, that will always happen. Each will always try to force their authority over the other. Impoundment is an excellant example.
JQ Adams was the first president to do it and all following presidents did it. It was always done in a small way and centered on how the prez would run his branch most effectively. Congress would bitch and moan but the infractions didn't rise to the point that justified a stand-off. This changed when Nixon got egregious. Congress took him to court and won. Thereafter, the ability of the prez to manipulate appropriations, even in a small way, disappeared.