A slice itself is just a window into a backing array of fixed size. The slice carries three data members. The pointer to the backing array and its remaining capacity and the length of the slice data.

Typically slices are passed around by value but you can take their address and modify a "shared" slice.

The built-in append() returns a new slice by value.

What happens is simply that when appending data to a slice and there is no room in the backing array, a new backing array is allocated that the returned slice points into. The old "input" slice to append is still intact and if some code has access to it, it will look at data stored in the old backing array.

I've constructed similar utility types in C and find them quite convenient. It's very convenient to have the distinction between the backing memory (array) and a slice viewing a portion of it instead of just a dynamic array.