TL;DR

Given two columns:

a b
---
0 5
1 6
2 7
3 8
4 9

create_array will combine these two columns into an array:

create_array(a, b)
------------------
[0, 5]
[1, 6]
[2, 7]
[3, 8]
[4, 9]

And of course, you can put create_array in another create_array:

create_array((create_array(a, b)), NULL, NULL)
----------------------------------------------
[[0, 5], NULL, NULL]
[[1, 6], NULL, NULL]
[[2, 7], NULL, NULL]
[[3, 8], NULL, NULL]
[[4, 9], NULL, NULL]

The memory layout of the last result is like this:

Array { 
    array: Nullable { 
        column: Array { 
            array: Int16([0, 5, 1, 6, 2, 7, 3, 8, 4, 9]), 
            offsets: [0..2, 2..2, 2..2, 2..4, 4..4, 4..4, 4..6, 6..6, 6..6, 6..8, 8..8, 8..8, 8..10, 10..10, 10..10]
        },
        nulls: [false, true, true, false, true, true, false, true, true, false, true, true, false, true, true]
    },
    offsets: [0..3, 3..6, 6..9, 9..12, 12..15]
}

The memory layout shown above is a three-layers indirect tree into a single underlying data slice:

[0..3             , 3..6             , 6..9             , 9..12            , 12..15               ] : which row? (the range points to the next layer)

[0..2,  2..2, 2..2, 2..4,  4..4, 4..4, 4..6,  6..6, 6..6, 6..8,  8..8, 8..8, 8..10, 10..10, 10..10] : offsets into a flatten array item (the range points to the next layer)
[false, true, true, false, true, true, false, true, true, false, true, true, false, true,   true  ] : is the item null?

[0, 5,              1, 6,              2, 7,              3, 8,              4, 9                 ] : array data

TL;DR

Define the function get(array: Array<T>, idx: i16) -> T for any T is just as simple as:

registry.register_2_arg::<ArrayType<GenericType<0>>, Int16Type, GenericType<0>, _>(
    "get",
    |array, idx| array.index(*idx as usize),
);

Then it will just work for any T, including multi-dimensional arrays. Explained in pseudo-sql:

create table t (arr Array<Array<Int16>>, idx Int16);
insert into t values ([[0, 1], [2, 3]], 0);
insert into t values ([[4, 5], [6, 7]], 1);

select get(arr, idx) from t;
+---------------+
| get(arr, idx) |
+---------------+
| [0, 1]        |
| [6, 7]        |
+---------------+