Understand Btc

Is there any single video that explains what btc is and how it is used.

I don’t understand btc, if its purpose is to replace the regular currency why does it go up and down like a stock that’s supposed to give returns. I don’t understand what are the practical use cases and how is it being used right now apart for illegal activities.

And who even takes care of the btc ecosystem and are they just random people, the concept of ledger everything is so confusing.

Help me understand, thank you!