A beginner and I’m so confused. How do i get to the fulfillment part?

I’ve done my due diligence, but i am still confused on how i retrieve my fulfillment center address. I also have other questions that need answers as well.

I am aware I’m supposed to create a product listing first. After i contact my supplier on, let’s say, alibaba do i immediately go to Amazon Seller and input as much information I can about the product I wish to sell?

Some of the question when you create an item is what I am confused about, “brand” is this not the brand name I created when I signed up to Amazon Seller or is this something where I can change my brand name to fit my niche?

Under brand it asks questions like #/ID Number, how do i even retrieve that information?

During product input, should i message my wholesaler to help me with certain questions or should i just get everything from the item description on Alibaba?

And for shipping details, how will I know the dimensions? Do i ask my wholesaler?

Afterwards, does Amazon automatically give me an address for a fulfillment center near me or would i have to pick for myself?