Please help me understand Blockchain Consensus

Watched many videos, read articles and yet I can't understand blockchain.

How transaction takes place?
Why there is there even a math quiz or problem attached, that miners need to solve to be able to verify a transaction?
What makes it "Proof of work"?
Why can't anybody hack it?

Please help me understand. These are the questions that I'm not able to understand. I feel like understanding the answer of any of the questions above can help me understand the whole concept.