I think you're describing an unrolled linked list: https://en.wikipedia.org/wiki/Unrolled_linked_list

Although specifically, in an unrolled linked list, the blocks are connected by a chain of pointers from one to the next. You can alternatively just keep pointers to each block in a top-level array. it's not clear from your description which you're doing.

The JDK has a twist on the latter, which it calls a spined buffer, where the blocks double in size every time one is added, which makes sense if you have absolutely no idea how many elements there will be when you start.