why do Russian and some other countries aircraft carriers have that scoop going uphill at the end of the runway, but ours stay flat the whole way?

Russian and some other nation's aircraft carriers use a "ski-jump" ramp (the upward slope) because they lack the complex, expensive catapult systems used by the US Navy. The ramp allows aircraft to take off using their own power, boosting them upward to gain altitude, whereas US carriers use steam or electromagnetic catapults to launch fully loaded, heavier aircraft from a flat deck.
Key Differences:
Russian/Ski-Jump Carriers (STOBAR): These use Short Take-Off but Arrested Recovery (STOBAR). The ramp gives planes extra lift at the end of the deck. This setup is simpler to build but limits aircraft to lower maximum takeoff weights, as they cannot carry as many missiles or fuel compared to a catapult launch.
US/Flat Deck Carriers (CATOBAR): These use Catapult Assisted Take-Off but Arrested Recovery (CATOBAR). Catapults (like on the Gerald R. Ford class) accelerate planes to high speeds in seconds, allowing heavily armed, larger, or slower-accelerating aircraft to take off.
In essence, the ski-jump is a substitute for the high-tech,, high-power catapult systems that enable the US to launch heavier, more heavily armed aircraft.