Short answers, pulled from the story.
The seven original electors included three archbishops and four secular rulers. These specific princes held the authority to select future emperors as identified by Pope Urban IV in 1265.
The Golden Bull codified these rights into law in 1356 after decades of dispute. This decree established the Archbishops of Mainz, Trier, and Cologne alongside the King of Bohemia, Count Palatine, Duke of Saxony, and Margrave of Brandenburg.
Eight electors now existed since Bavaria retained its original seat while a new electorate formed for the Count Palatine. Two Wittelsbach lines remained estranged enough not to threaten combined power following this arrangement.
Treaty of Lunéville ceded Rhine territory to France leading to abolition of Trier and Cologne archbishoprics in 1801. Mainz transferred its status to Regensburg becoming prince-archbishop there before Napoleon created four new electorates including Württemberg Baden Hesse-Kassel and Salzburg in 1803.
The Archbishop of Mainz served as Archchancellor of Germany while Cologne handled Italy and Trier managed Burgundy. These offices were discharged only during coronations where they bore crowns and regalia.