Shipping and Amazon warehouse FBA confusion

I’ve been doing research, but I’m still very confused. I just need some assistance getting the right footing.

My question involves FBA. So for example, I choose a product, then find the product on a wholesale website, like Alibaba, for example, how do you know which Amazon warehouse to ship the product to? Will any Amazon warehouse know to expect a shipment from you? Do you have to notify the warehouse in advanced you’ll be storing the product there? I tried to find this in the seller dashboard, but it’s very unclear to me.

Also, if you’d like to send a sample of the product you purchase from Alibaba, or wherever, how would you go about only shipping one or a few of the products to your house?