1700's and 1800's Adoptive Children

Reading thru church marriage records from the 1700's and 1800's, I find myself wondering who could of adopted children that had no parents. I have seen plenty of marriage records which read "hijo adoptivo" or "hija adoptiva".

My question is:

Would these children most likely be adopted by the "Padrinos" or would they be taken in by immediate relatives.

I have a marriage document for one of my ancestors that indicates that he was and adopted child and he went on to carry the adoptive mothers surname and not the adoptive fathers surname.

Is there any chance that a male adopted child would take on the mothers surname (adoptive mother) instead of the adoptive father so as to continue his "real" surname.

This action makes me wonder if his adoptive mother (aunt, madrina etc) was directly related to his real father and therefore made it easier for him to carry his real surname.

How common was adoption during the 1600's, 1700's and 1800's ?

Were padrinos held responsible for fulfilling their obligation of taking care of a child that lost his parents?

Why does a marriage record of an adopted child not indicate that the parents were "difuntos" and list the parents by name and then list the adoptive parents by name.

This kind of leads me to believe that the child lost his parents at a very very young age and it was best to honor and list the adoptive parents who raised the child from birth.

If the child was an illegitimate child the marriage record would say "padres no conocidos". Right?

Hope someone can shed some light on this topic