This code appends to the existing s slice a new element. It uses the first grayed element (which was allocated but not yet used) to store element 2, as figure 3 shows.
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Because an array is a fixed-size structure, it can store the new elements until element 4. When we want to insert element 5, the array is already full: Go internally creates another array by doubling the capacity, copying all the elements, and then inserting element 5. Figure 4 shows this process.
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First, s1 is created as a three-length, six-capacity slice. When s2 is created by slicing s1, both slices reference the same backing array. However, s2 starts from a different index, 1. Therefore, its length and capacity (a two-length, five-capacity slice) differ from s1. If we update s1[1] or s2[0], the change is made to the same array, hence, visible in both slices, as figure 6 shows.
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The shared backing array is modified, but only the length of s2 changes. Figure 7 shows the result of appending an element to s2.
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This code leads to creating another backing array. Figure 8 displays the results in memory.
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