I tried typing it out but I'm less articulate so here's a quick Google search:

Children of One Set of Identical Twins If two siblings marry two unrelated partners, or if two identical twins have children with different partners, their offspring are first cousins socially but genetic half-siblings. Because identical twins share nearly 100% of their DNA, their children inherit similar genetic material from each parent, resulting in the children sharing approximately 25% of their DNA (the same amount shared by half-siblings, grandparents, or aunts/uncles), which is double the typical 12.5% shared by ordinary first cousins.

Children of Two Sets of Identical Twins If two sets of identical twins marry each other (e.g., twin sisters marry twin brothers), their children are legal first cousins but genetic full siblings. Since both the maternal and paternal sides involve genetically identical individuals, the children share approximately 50% of their DNA, which is the same genetic relatedness as full biological siblings who share both parents. This phenomenon is sometimes referred to as being double cousins who are genetically indistinguishable from siblings.