Scorestreaks provide a variety of automated and manually controlled battlefield weapons, ranging from intelligence gathering to outright enemy team destruction.
You can set any mix of three Scorestreaks before entering a match, and they are earned by acquiring enough score without dying.
This makes expensive streaks difficult to earn, especially if you are a newer player, and doubly so when playing aggressively or near objectives.
In general, playing defensively or conservatively is necessary to unlock high end streaks, but as your skill in Infinite Warfare improves, you should be able to aim for mid to hi-end streaks more often.
Another option is to take the Drone Package and hope for a bit of luck with care package rewards. This is not a good choice if you’re trying to run with a specifically focused streak loadout for a certain game mode, but otherwise, it’s fun to use and can occasionally gift you with some powerful streaks.
Don’t mistake low cost streaks for weak streaks. Likewise, earning a T.H.O.R., R-C8, or Apex once every few matches isn’t going to cause nearly as much damage to the enemy team as calling in multiple UAVs every match.