Welcome to the Forum.

I believe all the players get some boost when you run a drill, with the larger boost given to the players you are controlling if you're successful in reaching the target score. To ensure you're developing your players at both ends of the court however, you should run offensive and defensive drills.
As far as I'm aware, the 5 on 5 scrimmage does not provide any ratings developments at all. It's just a means of getting to know the players on your team. It's worthwhile doing to familiarise yourself with your players' abilities and shooting form (and thus the release point when you're shooting jumpers with them) but apart from that, it won't improve your roster.
Players have a hidden development rating and older players are much less likely to see improvement compared to young players, so results will vary player to player. If you want to develop a young player then it's important you consistently select them when running the drills. All players develop or decline at the end of the season but when developing players in the Academy during the year, they must take part in the drills to see any change in their ratings. You'll soon find out which players have the best development potential, though once again they will be the younger players on your team. You're not going to get much development out of a player like Allen Iverson at this point in his career.
Hope this helps, feel free to post any further questions you might have.