>tried to put him in jail

They really didn't. It was a dog and pony show under the belief that he would not make his way back into power. The dems/reps did not want to set a precedent of holding a president to account for doing terribly illegal things. They didn't intend to actually do anything to prevent this.

And so here we are.

The GOP in Congress abdicating its role and deferring to the executive, as well as SCOTUS continually using the “shadow docket” to rule in his favor with little to no explanation provided.

Congress: anyone falling out of line will lose his support in midterms.

Judiciary: appointments and ideological alignment with some of the Supreme Court. Thomas and Alito are fully controlled, Kavanaugh just loves a powerful executive, the rest aren't controlled but often in agreement.

Then there's his use of executive power to punish his adversaries, e.g. Perkins Coie.

The question is through what mechanism are other branches curtailing his power? It seems to be limited to strongly worded letters and speeches, indignant comments and scathing news reports but nothing real.

Through the flawed primary system. Relatively few people vote in the primaries, which means they skew towards extremists. Trump can motivate MAGAns to vote in Republican primaries, which makes MAGA essentially a gatekeeper to Republican seats even in districts where the electorate at large is Republican rather than MAGAn.

He's not directly controlling the judiciary yet, but he has appointed wildly extremist judges and threatened judges who rule against him with impeachment, so he's certainly making an effort.

The execution of the Unitary Executive theory, a clear ideological descendant of Carl Schmitt’s Decisionism. Carl also had some beautiful prose describing the weaknesses of liberal democracy and how to exploit them that are very relevant to today as well.