The state is divided into districts and people run in that district for the office.
Representatives are elected by the people in a state.
By the legislatures of the different state. Each state government chose their representatives to the Constitutional Convention
Senators are the members of the House that are chosen for a state's entire population. Representatives are chosen to represent a portion of their state.
legislators
legislators
constituents
At-large.
The House of Representatives are chosen based on a state's population. The number of representatives each state has is subject to change.
House of Representatives (435 representatives) chosen through proportional representation. Senate (100 senators) chosen by equal representation, 2 senators for each state.
If there is no majority in the Electoral College, the House of Representatives chooses the President. If there is a tie in the College, one of the two people tying must be chosen. Otherwise, someone from the top three must be chosen. Each state gets one vote. The representatives from each state meet and decide what their vote will be.
The House of Representative numbers are chosen by the population of the state. Each state gets one representative in the house for every 40,000 inhabitants of their own state. The Senate numbers aren't based on population. Every state gets 2 representatives in the senate.
Under most circumstances Senators are chosen by popular vote in a State wide election in the State they represent. Occasionally when an incumbent Senator dies or otherwise leaves office the Governor of their State will choose a temporary replacement, depending on the laws in that state the replacement will finish the term of the exiting Senator or will serve until a special election is held. US Representatives are chosen by popular vote in the congressional district they represent. The manner in which they are replaced varies from state to state.