What exactly are birth marks and we do we all have them?

Title says it all. What's up with birthmarks?
Birthmarks are a type of discoloration that appear on the skin at birth or during infancy. [1] They can occur anywhere on the face or body, and some are permanent while others may fade away. [1] While the underlying reason for why birthmarks form is unknown, some doctors believe that a tiny piece of placenta may become lodged inside the developing embryo very early on in the pregnancy, causing damage to the nerves that control the widening or narrowing of capillaries. [3] Some experts believe that some proteins produced by the placenta during pregnancy may be linked to a higher chance of developing some types of birthmark. [3]
Birthmarks are common type of discoloration that appears on the skin at birth or during the first few weeks of life and they’re usually noncancerous [1, 2]. They can occur anywhere on the face or body and vary in color, size, appearance, and shape [1]. Some are permanent and may get larger over time while others fade away completely [1]. Birthmarks are thought to be caused by buildup of cells that line the blood vessels of infants, which may cause strawberry marks to occur [2, 4]. Some doctors believe that a tiny piece of placenta may become lodged inside the developing embryo very early on in the pregnancy, resulting in port-wine stains especially if the capillaries permanently widen in one area [2]. Also, many experts believe that some proteins produced by the placenta during pregnancy may be linked to a higher chance of developing some types of birthmark [2]. Very rarely, some birthmarks are caused by gene mutations, such as Sturge-Weber syndrome, which appears as a port-wine stain and is caused by a different gene mutation [4].