Adding an array to an element within an array

[charity]

Greetings. I currently have an array, $arr = [ 'Id' => '1', 'Name' => 'sadiq', 'List' => [] ]. As you can see, 'List' is an array. I want to add a separate array to this list. How can I do this? My other array is $arr2 = [ 'Letter' => 'c', 'Color' => 'red' ].