It's very illegal. He has already been impeached once for withdrawing military aid to Ukraine in his first term.

And you see where that got us.

It isn't daft. The Republicans since Reagan have pushed a fringe legal theory called the Unitary Executive Theory. Basically, they want the president to fully control the executive branch and military such that theirs is the only voice that matters for much of the government. Not unlike a king, but partially checked by congress and the courts. They have been taking (illegal) actions to try to get sued, and also have been suing others/other branches of government, to try to get the Supreme Court to hear cases that will support this fringe legal theory so that it becomes the law of the land.

I am not a lawyer, but this is possibly something Trump can legally do since he is Commander in Chief of the armed forces. However, this seems more like an apportionment thing, which is Congress' responsibility. Congress has allocated funds to send military aid to Ukraine. So, even if Trump as Commander in Chief could say "no more weapons to ukraine", it seems doubtful to me that he could (legally) stop weapons shipments currently en route.

But, by the time whatever government office sues the office of the president to get a judge to enjoin them to send the agreed upon weapons that were already apportioned, it will already have hurt Ukraine somewhat. Trump often weaponizes inefficiency. And these sort of illegal acts aren't crimes per se - they're just procedural breaches - the legal remedy is just to reverse the action.

So, probably not legal. But Trump gets to weaponize his administration's incompetence (or feigned incompetence) to at least delay aid. More competent people may support these actions, knowing they're illegal, to try and strengthen the president's role even further.

Republicans hold the majority in congress, so they're refusing to push back and are just allowing him to usurp power from them.

It is for Congress to decide such matters. It says so right in our constitution, but our Legislative branch has been ceding their ability to check Executive overreach for decades. Now, with a complicit Congress as the majority, they will never challenge Trump on anything he does, no matter how unconstitutional or illegal it might be. Same goes for the Judiciary as well, chock full of partisan hacks who bend and twist their interpretation of the language of the constitution to cherry pick a favorable ruling for anything that Trump does.

Basically, our system of checks and balances that are meant to keep one aspect of the government from becoming too powerful has been completely subsumed by ideologues who prefer monarchy to democracy. The executive now essentially has unchecked authority and is in full control of the government apparatus.

how is Trump so powerful?

The Republican party controls Congress and they have abdicated their responsibilities to our Constitution and, by extension, the American people in favor of an authoritarian because they have accepted him as their leader, thus giving away their own power.

Our forefathers never expected Congress to give away its own power. Because it doesn't really make any sense in the long term for them to do so.

We are experiencing a coup. Most Americans are too uneducated to understand this.

We are entering a "yeah and who's going to stop me era of US politics"

The president was always able to stop aid under certain conditions. Trump is just going to certify that one of those conditions exist no matter how ridiculous it is.