At some point a drill is not useful anymore and the player gains very little. It's why you need a variety of drills and then do them in a certain order to get some skills very high.
For example you train a single drill to average 120% and then proceed to the next one.
If you have player with 60% every skill to start with, you do shooting technique to 120%, the player will then have shooting, finishing and strength at 120% and everything else at 60% still. Then you do pass go shoot you can get shooting way beyond 120% as passing and speed are still at 60%. It is how you build a piece, a player. You think about what attributes you want real high and then use different drills in an order to get there.
The higher the attributes of one drill are, the less effective the drill will be. So having only pass go shoot and fast counter, will leave you with almost no potential to create a good player... In the first seasons it is not that important, just learn the game and try to save some rests and token on your way.
For Goalkeeper there is a similar strategy to create one. Goalkeeper training is the basic drill, I always do first. There are many possibilities to create your player and choose which skills will be the highest, it depends on your order of training.