Is It 2Nd or 2Th?

In this lesson, we're looking at the spelling of ordinal numbers and a few rules. 1st = first (She won first prize.) 2nd = second (I live on the 2nd floor.) 3rd = third (Take the third turning on the left.)

Which is correct 2nd or 2th?

There is no "2th". That is incorrect. You're probably looking at "12th". This is the only case when "th" follows a "2"

What is correct 23th or 23rd?

It's 23rd. In the US we normally say "October 23rd (or just "23"), 2019," but UK usage is probably different.