I wouldn’t call it an average. I think of it more of a composite.
And yes I think it’s calculated the same way for all players and not weighted for position. I’ve seen pitchers, for example, with higher Overalls than I expected and then realized they’re very good fielders.
Another example, Catchers kind of have a ceiling on their Overall because they naturally have low Range and Glove, unless they’re DiTR in which case their OA skyrockets because they can play other positions too.
As always, OA is skewed heavily toward Dur and Sta. Whenever I’m looking at draft prospects and I see a player with a high OA who can’t hit and isn’t a SS, I know he’s got a DUR over 85.