I understand the question is that alcohol gets into the breastmilk - so why doesn't it stay there, hanging about, waiting to 'get into' the baby at the next feed? Well, I have to say the exact biochemstry of this is miles over my head, but apparently what happens is that unlike (say) the urine in the bladder, 'alcohol is not trapped in breast milk, but is constantly removed as it diffuses back into the bloodstream'...this is from this scientific answer which goes on to explain more about the timing of the whole process.
Pumping and dumping does not hasten the process of elimination, and in fact it is a pointless thing to do - you just hafta wait until your body goes through the process at the same speed as it gets rid of alcohol in the bloodstream (via, I suppose, the liver)....so alcohol in the milk diffuses back into the bloodstream, hurtles round the body in the circulation, and is dealt with in the normal way as you de-toxify and become alcohol free.
Some components of some foods do get into the breastmilk, in varying amounts.
I am not sure exactly what is in breastmilk, and nor is anyone, as far as I can tell. They keep discovering new stuff!
I am not sure how it is made - this says something about it but it is all v. technical. The basic thing is having been pregnant and having breasts, but there are many cases where adoptive mothers have induced a milk supply, and one or two where men have got some milk, too, simply by having the baby suck on the breast.