The House proposes it, and theyāve just voted that they agree it (Democrats have a majority there). It then gets passed to the Senate, who are supposed to hold the trial, and it has to get 2/3s of Senators to vote for it to get through. But itās split 50-50 Dem-Repub there, so itās less likely to happen.
Also the setup is really unfair- Representatives in the House are like MPs- they represent a district with so many people (even if some of the deciding of districts is really corrupt). So for example California has more Reps than Montana, because California has 40 million people and Montana has less than 1 million. So California has 53 seats in the House and Montana has 1. Democrats usually have the majority here because they are popular in the places where people live.
However in the Senate, every state has 2 Senators, no matter how big or small. This gives the Republicans an unfair advantage because they can get senators for rural states with hardly any people, and they get a higher representation than the democrats with less voters. This is also why theyāre against Puerto Rico and Washington DC becoming states. Theyād get senators too then, and the people in those places will pick Democrats (also very large black and latino populations in those places)