Is the whole jacket lined with sherpa (i.e. fluffy) or is that just on the collar?

All of the inside is lined with the "sherpa' except for the sleeves. Note the "sherpa' is made from RECYCLED plastic bottles and is NOT white. Is a slight antique-white color.

I know the sherpa doesn't run throughout the arms, but the arms are insulated and not just denim, correct?

Yes, the arms are lightly insulated with a quilted poly material

I’m 5’6” and 210 pounds. i usually wear a xl but can wear larges. i just like my clothes to fit a little bigger. what size jacket should i get?

I'd go with the XL here as well. I'm 6'1, 192-195 and XL is perfect for me. General thought is just wear one size up from the shirt size you wear, which is Large for me.