If $C = \{0,01,011\}$, then $C_\infty = \{1,11\}$ which is disjoint from $C$. It follows from the Sardinas-Patterson Theorem that $C$ is uniquely decodable, as we have already seen.
What is the procedure to get from $C = \{0,01,011\}$ to $C_\infty = \{1,11\}$ ? Thank you
Proceeding as described here,
$C_1=\{1,11\}$ (the result of removing a code word, when possible, from the left of any different code word)
$C_2=\emptyset$ (Nothing in $C_1$ begins with a code word, and no code word begins with anything in $C_1$.)
$C_i=\emptyset$ for $i>2$.
I’ll guess that $C_\infty$ means what it does here: the union of the $C_i$ for $i>0$, which in this case is $\{1,11\}$ and is disjoint from $C$.