Player and team boosts include
Sprint Speed
Acceleration
Strength
Tackling
Passing
Shooting
Dribbling
Heading
Crossing
Free kicks
Jumping
Aggression
include
Soccer Cleats
Socks
Shorts
Shirts
Equipment boosts
Ball
All these are earned through day to day play of the game. A player earns random chests of different colors (bronze, silver, gold as an example) with various boosts. To accelerate a player improving their team chests can be purchased for immediate use in the game.
Also the higher level your team becomes additional playing stadiums open up with more opportunity for better chests earned or purchased(platinum, diamond etc.)