Extend Relationship in Software Engineering subject

Hi everybody. I had a problem while I was learning about Use Case Diagram as follows:

My teacher said that First, if UC B is an extension of UC A, then UC B must be attached to an Actor. If UC B is not attached to an Actor, it is wrong. Second, if UCA includes (UCA —included–>UCB), then at least one UCC must also include UCB (for example, UCA —included–>UCB and UCC —included– >UCB ), if only (UCA —included–>UCB) is false.

However, when I read on websites, combined with watching videos on YouTube, everything is not that complicated. Can the engineers here tell me which is correct?