Where should the Chazan stand in Shul and where should you read the Torah from? What is the source?

The Sephardic custom is that the Chazan stands on the raised platform in the center of the Shul. This custom appears in Sefer Hamanhig who explains that it is based on the Talmud Succah 51b. The Talmud describes that in the Shul of Alexandria, Egypt there was a wooden raised platform in the middle and the Chazan stood on it, and when it was time to answer Amen, they would wave with a cloth do to the huge size of the Shul. The Ashkenazi custom is that the Chazan stands in front of the Amud near the Holy Ark. The custom should not be changed and the Chazan should not stand on the raised platform in the middle of the Shul unless it is an unusual case were there are very many people, and the Chazan will not be heard if he does not stand at the Bimah. This custom appears in the Tur and is explained at length in Igrot Moshe. The Magen Avraham writes that nowadays the custom is that the place were the Chazan stands is lower than the rest of the Shul, because of “Mima’amakim Kraticha Hashem”, and that is the reason the Chazan is always called “Yored Lifnei Hateivah” [goes down in front of the Teivah]. This Halachah is also quoted by the Misnah Berurah, but nevertheless the Igrot Moshe explains that the custom in most places is not like the Magen Avraham but like the Shulchan Aruch and Rema that don’t require a low place. The custom of all is that during the reading of the Torah the Ba’al Kore stands at the elevated Bimah in the center of the Shul. This custom appears in the Rema, and the Biur Halacha adds that it is an ancient custom that is explained in the Rambam and the Tur, and the source is in Succah 51b as the Gra wrote. Unfortunately this ancient custom was breached in some places, where they started placing the Bimah near the Holy Ark, because they wanted to follow the way of the idol worshipers, as they do in their temples. The latest Poskim spoke against these people at length. [רמב"ם הלכות תפילה פרק יא הלכה ג, ספר המנהיג דיני תפילה אות סז, טור או"ח סימן קנ, שו"ע סימן צ סעיף א, מג"א ס"ק ג, ומ"ב ס"ק ה, רמ"א סימן קנ סעיף ה, ומ"ב ס"ק יב ויג, ובה"ל ד"ה באמצע בהכ"נ, אג"מ או"ח ב סימנים כו וכח, ובאו"ח ג סימן י].