Ah, well, while they all have associated symbols, they don’t really manifest physically. You can occasionally see said symbols manifest in light whenever someone’s crest activates, and if you have equipment that detects crests it will show the symbol, but for the most part, it’s simply a characteristic inherent in somebody’s blood. Many of Fodlan’s noble lineages have crests associated with them, and they are considered a very desirable trait to have when it comes to arranging marriages and the like.