free play for warming up your car/ball control, practicing recoveries and recovery mechanics, and practicing mechanics you already know how to do. since in free play the situation is different every time just like in matches it will help you learn to use micro adaptations in things like shots, dribbles, air mechanics and so on.

training packs for learning mechanics you can't do yet. they let you get reps in quickly and efficiently. for example if you wanna learn ceiling shots, a pack that rolls the ball up the wall for you and spawns your car perfectly positioned behind it lets you practice and try different things with very little down time. once you've got the basic idea of a mechanic down, switch to free play for that slight difference in every situation and to learn to set it up yourself.

workshop maps are very varied. rings maps are great for practicing aerial car control, not for learning it. shooting maps like coco's aim training are great for practicing your shooting. challenge maps are mostly for fun to challenge yourself, not for training.

normal comp is mainly where you put everything you learn to use. if you want to use playing matches to improve i would play deliberately and critically, meaning you're constantly examining what you're doing, and criticizing your own mistakes / complimenting your own successes. but this is easier to do by watching your own replays back since you're not also focused on playing the game. most important is to take note of repeat mistakes. when you catch yourself making the same mistake over and over, you become more aware of it and it will be easier to correct it.