What exercise hits this spot on my arms?

[Huanbo]

I've been workout for 3 yrs, my arms are thick and stuff, but I just can't see this "separation" on my arms! What should I do about it? Is it because my bf is too high?(My bf is 13%) And what exercise hits this spot? Thanks.

I already circled the spot I am talking about with red pen. And another pic is my current arms pic. Idk if it is because i need more size to show that " separation" or cut down on bf.

Thank u for ur help guys.

[Chevyrado06]

Delts delts delts. Hammer your medial and rear delts

Lower body fat will help.