y'all please forgive me if this has been discussed before,
but after a look-around at this forum, I found too many
casual spoilers to dig any further until I've played the
game all the way thru atleast once ...

but this little thing has bothering me for awhile now ...

when I drink a soda-pop, I see the bottle-cap added to
my inventory, I see the Aid-benefit take effect, but where
does the empty bottle go ?

On the ground.

When you finish drinking, the bottle turns to dust and a gust of wind blows it away. It just disappears.

The bottle-cap is added to your inventory because it's either basic currency (the game assumes you'd do the logical thing and keep what's basically $1 rather than asking you if you'd like to keep $1 after every bottle, which is a no-brainer... ) or certain ones are part of a quest. I won't say more than that, and don't worry, because the way the quest is introduced, they basically shove it in your face along the way and announce those are important (if they haven't already), so I haven't really spoiled anything for you. The quest broadcasts itself and makes SURE you know certain bottlecaps are important, it's no secret.

As for why the bottles aren't added to your inventory, I'd imagine this is a leftover from FO3 to New Vegas. In FO3 they probably decided it'd be tedious to give you empty bottles to toss out (not to mention taxing on the game engine, encouraging you to randomly distribute litter throughout the world), so the implication is basically that your character automatically tosses the bottle. In New Vegas, empty bottles CAN actually be put to good use, but they either didn't think of changing it, didn't find it worthwhile or, again, found it too taxing on the engine to implement such a feature that encourages the player to litter.

It shouldn't really be taxing on the engine. A script can be attached to the drinks that adds the appropriate bottle to the player's inventory on use. It think it was just something that was so low-priority, they never bothered to implement it.

The bottle disappearing is a bug, what's supposed to happen is that the Courier breaks the bottle and its gets added a melee weapon.

Why do I randomly have used cartridges from guns I use? Why can I carry 300 lbs of guns, ammo, gear, etc? How come I can carry 5000 rounds of 5.56 and have a magazine ready for each one?

Because it's a videogame and not reality. There are so many things that you'd do in real life that aren't in the game. Can you even carry 5 glass sodas around, while trying to shoot people and the shot at yourself? No, you can't.

And what's the point of having so many bottles? It would cause a laggy game eventually because sodas are EVERYWHEREEEEEEE!!!

.2c