Understanding the use of 'it' for time, day, month, date, weather, and situations

How is 'it' used in the sentence 'It's a clear sunny day'?

Which sentence uses 'it' to describe someone's situation?

The sentence 'It's an old school and quite famous' uses 'it' to refer to:

Which sentence from Angshu's mail correctly uses 'it' to state the time?