Overall, the Thanos movie has some similarities to the comic, but there are differences. Some parts are adapted to fit the movie format and appeal to a wider audience. But the core of Thanos' character and his goals remain similar.
Sure. In the movie, Ant - Man was stuck in the Quantum Realm while Thanos was on his rampage collecting Infinity Stones. Thanos' actions led to the Decimation, but Ant - Man's time in the Quantum Realm meant he was spared from the snap. His return was significant as it offered a new approach to fighting Thanos. The Quantum Realm had its own strange physics and possibilities that Ant - Man could utilize against Thanos.