Short answers, pulled from the story.
Three men held the office of Reichspräsident under the Weimar Constitution: Friedrich Ebert, who served from 1919 until his death in February 1925; Paul von Hindenburg, who succeeded him and served until his death on the 2nd of August 1934; and Karl Dönitz, who was named president by Hitler in April 1945 and held the title until British forces arrested his government at Flensburg on the 23rd of May 1945.
Article 48 of the Weimar Constitution allowed the president to deploy armed force, suspend civil rights guaranteed by the constitution, force uncooperative state governments to comply with federal law, and issue emergency decrees that carried the same legal weight as legislation passed by the Reichstag. All decrees required a counter-signature from the chancellor or a competent minister, and the Reichstag had the right to reverse any measures taken under the article.
On the 1st of August 1934, Hitler's cabinet issued the "Law Concerning the Head of State of the German Reich," which merged the offices of president and chancellor into a single role to take effect upon Hindenburg's death. Hindenburg died the following day. Hitler then styled himself Führer und Reichskanzler and declined to use the title of Reich President. A popular referendum held on the 19th of August approved the merger.
Ebert invoked Article 48 on 136 occasions during his presidency. He consistently based each invocation on agreements between himself, the government, and parliament rather than acting unilaterally.
Hindenburg appointed Hitler chancellor on the 30th of January 1933 under pressure from his advisors and citing Germany's continued economic and political instability. When Kurt von Schleicher, the outgoing chancellor, asked Hindenburg to declare a state of emergency to prevent Hitler's appointment, Hindenburg refused, calling the move clearly unconstitutional. With his own political support exhausted, Schleicher resigned on the 28th of January, leaving Hindenburg with few alternatives.
The Weimar Reichspräsident was directly elected, held supreme command of the armed forces, could dissolve parliament at will, appoint and dismiss governments without a parliamentary confirmation vote, and issue legally binding emergency decrees. The Basic Law created the Bundespräsident as a chiefly ceremonial office largely devoid of those political powers, a deliberate reaction to the way Article 48 had been used to erode democratic governance.