Need Help appending / adding collections to an array

Hello, I am trying to take in an array of collections from a data store and batch them into arrays of a pre-determined size (partitions).

I have my scenario logic set with 2 repeaters, one counting partitions with an inner one counting individual array items like a nested for-loop situation. The array items themselves are collections.

In my example, I am sending in 5 collections that I am storing in groups of 3. EG - the first 3 should get stored in the first array/partition, the last 2 get stored in a second.

I create an empty array to initiate a partition, then try to add the first three input collections to it → this is the step I need help with. I am trying add() and merge() functions which are resulting in each of the input collections to be added to the data store as their own individual arrays as opposed to my desired result of appending them to the partition array that I created.

After the first set of three, the loop goes back and increments the partition number, creating a new empty array for the next set. As before, instead of appending to the empty array, new arrays are created.

I’ve attached a short (~3m) video showing my setup and logic, would be grateful if anyone would take a look and point me in the right direction, I’m basically looking for an equivalent to .push() in javascript.