The US system is based on English Common Law. (Except for Louisiana which actually has Napoleonic Law influence)

The Judiciary in the US is supposed to be impartial and apolitical. It is impossible to achieve full impartiality, but we have some rudimentary structures to make the system more impartial.

For the federal system the President heads the executive branch. The Attorney General (AG) heads the Department of Justice (DoJ) and the lower ranked prosecutors are internal to the Department of Justice.

The FBI and other law enforcement branches of the Federal government are also under the DoJ umbrella. They collect information on suspected crimes and give reports to the AG’s office. The AG’s office then files a criminal action against the suspected perpetrator in US District Court.

The suspect/ Defendant can hire an attorney to represent them or be appointed an attorney from the Federal Public Defender’s Office if they qualify. (Public defender’s are also under the executive branch.)

Judges (and the judicial branch) are more like referees who are there to make sure everyone follows procedure. In the US a jury must convict you unless you waive that right. So, the judge doesn’t even get a say in whether you are convicted unless something has seriously gone wrong. (JNOV/ directed verdict)

Kamala Harris was a States Attorney (District Attorney) in San Francisco in California and later the AG of California. California has a similar set up to the federal system, but replace the president with the governor of California. The State’s justice departments do not answer to the federal DoJ.