append Usage
func append(slice []Type, elems ...Type) []Type
The append built-in function appends elements to the end of a slice.
builtin package - builtin - pkg.go.dev
Example
Add a item to slice
var s []int
s = append(s, 1)
fmt.Println(s) // [1]
Add multiple items to slice
var s []int
s = append(s, 1)
fmt.Println(s) // [1 2 3 4 5]
Add another slice to slice
var s []int
s2 := []int{3, 4, 5}
s = append(s, s2...)
fmt.Println(s) // [3 4 5]