the two branches are theoretically independent. what is happening right now is that, depending in your POV:

1) the Republican majority in both the house and senate are entirely on board with all of the Trump administration's actions, and thus have no reason to pass legislation or act in any way separately from the executive branch.

2) the Republican majority in both the house and senate are craven and terrified of the MAGA base that Trump commands, and in many cases in the house, are very much connected to it, so much so that they will do nothing to stand up to the executive branch even when it has crossed lines that have not been crossed before in many different areas.

Take your pick.