I need help Tips/Tricks

How can I get better at call of duty? I’m an average Diamond player but I do not understand how people can fly through diamond and crimson. ( I know a team helps ) but people solo Q to crim-iri all the time. How can I get better at understanding Spawns, pinching, Timings, spawn blocking, even getting better aim, counting players. Are there any scrims for just average players? What can I watch or what can I do to actually get good.