What is the 2's complement answer of 16.5? According to this post it is saying Two's complement is only for integers, but in Wolframalpha is is saying the Two's complement of 16.5 is 0010000.1, how?
It seems that WolframAlpha is using fixed-point representation. In effect, it is doubling your number and then representing it as two's-complement.

In more detail, WolframAlpha is guessing that you are interested in fixed-point representations of the form
$$
xxxxxxx.x
$$
in binary, that is, 8 bits in total, with the "decimal" point at the fixed position between the 7th and 8th bits. Using two's complement, you can represent all numbers from $-64$ (represented as $1000000.0$) to $+63.5$ (represented as $0111111.1$).