Can somebody help me to figure out how this blueprints works?

Hi there, I am following this tutorial and stucked in the Tree’s blueprint.

You can check Create Branch Variants block in the BP below.

which I cant figure out is how does he add an vectors array into a sturcture array( Branch Variants ).This operation really confuse me quite a lot of time.And I also want to know the detail of sturcture Branch Variants.

Can somebody help me