confusion about append() built in

func append(slice []Type, elems ...Type) []Type

Does append ever update slice? or create a new one and copy and then return?

on the same track,

var slice = oldslice and copy(slice, oldslice) as slice is similar to pointer, I am not sure the difference

Either, depending on whether or not it's able to grow in place. Don't count on it always being different, and don't count on it always being the same.

I wrote an article about slices (https://appliedgo.net/slices/) that includes a visualization of the append operation, hope that helps. 

TL;DR: append updates the slice as long as the underlying array has some space left. Else it allocates a new, longer array. If you want to avoid or minimize copying and allocation, make the initial capacity large enough.

An interesting note: append modifies the slice header, which means the underlying array might share the same data, but the new value is a distinct slice value. This means you can do this: 

first := []string{"foo", "bar", "baz"}
second := append(first, "foobar")
fmt.Printf("%v\n", first)  // [foo, bar, baz]
fmt.Printf("%v\n", second) // [foo, bar, baz, foobar]