Devs, Officials, please explain how this game's training sector works in detail.

[khris]

Here I dont have the formula right now but keep in mind that white skills gain double XP, then theres a reduction on xp gaining that is around a 15% once you win a star(*) X number of skills that the drill affect.
Time ago you it was linked to the main star so once you reached 80, 100, 120... The gaining had a reduction but now for ex. If a drill affect x2 skills at 120, -so 60 x2- 160 so80 x2... 200, 240 etc theres a reduction on xp gaining. Remember that every player gains xp differently based on age slab and too if he's well programmed as fast trainer. Thats why you have to test players once you sign them, train a role or SA and check the gaining so to know if hes fast trainer and too, test the team vs a 1"% team so this align all your players at the 9* and you can compare a new signing, 4* 5* etc VS your best AvQ players by doing that.

[LUFC_MOT]

I try the same drill 4xLongRun on a fast trainer, academy player, old player, 80 average white skill, 100 average white skill, World Class drill, 2-star drill, ALL gain the same 12 XP.

I dont look at xp when training, just the number of skill gain for 1 rest pack.

Remember younger player gain more skill
18-21 100%
22-25 50%
26+ 25%

Fast Trainer Skill Gain
40-59 5.50
60-79 5.25
80-99 4.25
100-119 3.25
120-139 2.00
140-180 1.00

Academy Player Skill Gain (Estimation for 80% price ratio, vary based on price compared to fast trainer price)
40-59 4.50
60-79 4.00
80-99 3.33
100-119 2.00
120-139 1.33

Assuming the player is less than 22yo fast trainer and for eg Fitness is 120 and Speed is 100 (both white skill), for each 15 condition, the player should be gaining 3.25 skill, and then result maybe Fitness 121.6 Speed 101.6.

Assuming the player is less than 22yo fast trainer with DC role and for eg Fitness is 120 and Speed is 100 (average skill quality is 110), for each 15 condition, the player should be gaining 3.25 skill, for each 2 white skill gain, you gain 1 gray skill, and then result maybe Fitness 122 Speed 101.

To get a streamline squad, target only white skill when age is 18-21, so when the player reach his prime age, you may have 200+ in defensive/attacking skills, and gray skill in minus. Having player in 1 Role only is also a bonus (less white skill to train, less green, lower team quality, higher performance)
example for
DC - Warm Up Gym, Press The play
DL/ DR - DefendingCross, LongRun, Carioca
AMC - BallControl, ShootingTechnique, SlalomDribble

18yo 40% with playstyle is the best candidate, with the right training regime, you can use the player for ~15 years (depending on the level you are playing), so for every 1-2 season, only need to concentrate training on 1 player.