These mid-tem elections - are they run or held in the same way as a presidental election?
Same rules (such as they are) apply or different rules?
Same groups run them or other groups?
In the US, voting is not mandatory. While some states require employers to allow time off, we do not have a national day off to vote. Coupled with the perception that the President has special powers to affect the price of eggs, gasoline, and interest rates, the average voter perceives a different cost-benefit to voting in midterms compared to the Presidential election. Therefore, turnout is lower in midterms. The demographics also shift; midterm voters tend to be more white, wealthier, and more educated.
Voters go to the same polling places. The same local infrastructure is there from a voter perspective. However, all US elections are decentralized--they are run by state and local authorities (like County Clerks) rather than a single national body.
Rules differ because the President is elected through the Electoral College, whereas midterms are decided by a direct popular vote. In the Presidential race, most states are "winner-take-all," meaning the candidate who wins the majority of the popular vote in that state gets 100% of its electors (Maine and Nebraska are the only ones that allow these to be split).
Residents of US territories, such as Puerto Rico, cannot vote for President and have no voting representation in Congress. However, if a Puerto Rican moves to and becomes a resident of one of the 50 states, they can vote in all federal elections.
Representatives (House) are elected by popular vote in their specific Congressional district. Senators are elected by a statewide popular vote.
Is it just your upper or lower houses or both?
100% of the House of Representatives is up for election every two years, which is intense relative to the time spent campaigning. There is usually a primary election to pick the party's nominee, followed by the general election. Where's the time to legislate?
Only 1/3 of the Senate is up for election every two years. This is because a Senator's term is 6 years, and the seats are staggered so the entire chamber doesn't change at once.
So, as you can see, our perfectly streamlined system ensures that our politicians spend 100% of their time campaigning and 0% of their time governing while our voters spend 50% of their time being confused and the other 50% of their time not caring enough. Just as our corporate overlords intended.