Summary
Judge Juan Merchan said that he will rule on the immunity claim fairly and impartially on September 16th, 2024, and if he rules against Trump, then Trump will be sentenced on September 18th, 2024. His sentence (prison time, a fine, etc) is up to the judge himself.
Trump wants to appeal the guilty verdict, but the appeal cannot take place right now. Plus he has 3 other criminal cases.
And yes, Judge Merchan will rule on the immunity claim fairly and impartially because if a judge anywhere in the United States rules according to his/her personal or political views, then that judge is obviously not fair and impartial.
Just as the 6th Amendment of the Constitution of the United States requires jurors to be impartial, the judge must also be impartial.