Are char8_t and char16_t dynamic in size?

Here is what I know:

• "UTF-8 is a variable-length character encoding standard used for electronic communication."
• Likewise, UTF-16 is a variable-length character encoding.

Here is what I wonder:

• Are char8_t and char16 in C++ are dynamic in size too (to fit in the generic behaviour of UTF-8 and UTF-16)?
• Will it ever happen that when I assign it a specical value, it automatically expands its own size to match?