How do attributes change - for instance, I presume that blocking drills in the practice session improves blocking, but do athleticism and work ethic change and if so, how?

Practicing certain areas of a player's game will help him improve his skills. See the following table for a general (not position specific) list of how each area may impact a player's game:                                                                                                                                                   

Generally, happy players (getting starts/playing time) will work harder during practice (work ethic improvements).  With regards to the offenses and defenses, the more practice and playing time a player receives under each set, the better the offense will function in terms of efficiency.