(Spoilers)Really enjoyed Heroes Rising but I'm confused about one thing?

The ending was perfectly done with probably the most OP sakuga animation I have ever seen and that music was just amazing.

Also was touched in the heart how Deku is willing to sacrifice his dreams for the sake of saving someone.

However what I'm confused is that he transfer OFA to Bakugo by just mingling their blood. If I'm not wrong DNA needs to be ingested by the other person?

If mixing of blood is all it takes cant anyone take OFA during a battle? Since this can happen.

Also in S2 Stain was licking/eating blood to paralyse heroes so when he "ingested" Midoriya's blood how come he didn't gain OFA?