StringBuilder grows to any size as you insert characters, while Span<> only ever points to a pre-allocated, fixed buffer. It will never grow.

The other two questions are just nonsense.

Edit: I will point out that there is an internal ValueStringBuilder that works on a span (and thus has a fixed maximum size), but is much more efficient within its limitations.

These are different. Each string is an array of characters. In the definition of a normal string, an array is defined and if you want to make any other changes, another array is defined and as a result, another memory is allocated, but the StringBuilder comes and creates a final array. This explanation was very simple and concise and without details, which is complete here

In the source code of StringBuilder, you can also see that it is using the same common types and don't work on the structure.

Span is a ref struct that is a completely different type and that is why it is not used everywhere.

Of course, Span is not only used for string. It can be done for any other immutable type. FMI