Does the <li> tag always have to be nested within the <ul> or <ol> tag?

Question

HTML

Answer 1

li : List item
ul, ol : Unordered list, ordered list

A list item belongs to a list, one without would be an orphan, I prefer to write it along : )

Answer 2

no, it doesn't. If you just write something like this
<li>Hello, world!</li>
it will just output as an unordered list