The people who designed the US government back in the 1770s had a deep distrust of mankind and so they developed a system of government that would be difficult for one person or group to take control over. The government consists of three branches: Legislative, Executive, and Judicial. Each branch can affect the decisions of the others in a system called “checks and balances.” For example, the Legislative branch – Congress — makes the laws, but the Executive branch – the President – can veto them. And the Judicial branch (the courts) can declare them unconstitutional.
The Legislative branch is divided into two parts, the House of Representatives and the Senate. Collectively they are the two chambers of Congress. The House of Representatives has 435 seats, which are apportioned according to population, with the caveat that each state gets at least one representative. Each state gets two senators regardless of population, meaning that there are 100 senators.
Bills can originate in either house, but they have to pass both houses to become law. This fact becomes particularly important when one political party controls the House (Democrats) and another controls the Senate (Republicans), as is currently the case. In that case, the two houses have to work out a compromise to pass any laws. Note that this is a feature of the system, not a bug – it was designed this way to prevent tyranny.
(The authors of the constitution never dreamed that an entire political party would become subservient to the president and therefore act not as a check and balance but rather as an enabler, as the US Senate has done for Trump.)
The constitution gives Congress the role of spending money, so although the President delivers a budget to Congress every year, it doesn’t matter as Congress decides on the spending.