Question:

Why do the leaders of the House and Senate determine what to vote on?

Is this tradition or is it established in law?

What are the duties of the President of the Senate, the Vice President

re:

https://en.wikipedia.org/wiki/Vice_President_of_the_United_States

https://en.wikipedia.org/wiki/Mitch_McConnell

https://en.wikipedia.org/wiki/Party_leaders_of_the_United_States_Senate

The Vice President of the United States (informally referred to as VPOTUS, or Veep) is a constitutional officer in the legislative branch of the federal government of the United States as the President of the Senate under Article I, Section 3, Clause 4, of the United States Constitution.

As the Senate president, the vice president presides over its deliberations (or delegates this task to a member of the Senate), but is allowed to vote only when it is necessary to break a tie.

The Democrats began the practice of electing floor leaders in 1920 while they were in the minority.

The Constitution designates the Vice President of the United States as President of the United States Senate. The Constitution also calls for a President pro tempore to serve as the leader of the body when the President of the Senate (the Vice President) is absent. In practice, neither the Vice President nor the President pro tempore—customarily the most senior (longest-serving) Senator in the majority party—actually presides over the Senate on a daily basis; that task is given to junior Senators of the majority party. Since the Vice President may be of a different party than the majority and is not a member subject to discipline, the rules of procedure of the Senate give the presiding officer very little power and none beyond the presiding role. For these reasons, it is the Majority Leader who, in practice, manages the Senate.