What the Hell are Druids?!

Okay, so I've been talking to a friend about Druids and we realised we don't actually know where Druids get their power from. It's nature yes but can someone elaborate? Like this is what we have ruled out;

• It can't be some divine figure otherwise they would just be nature clerics.
• It can't be Nature spirits because nature spirits are primarily Fay and if it came from them they would be a Fay warlock.
• It can't be something you just learn through study and practice otherwise wizards would be able to do it or at least they would use INT for spellcasting.
• It can't be some kind of innate ability brought on by their heritage otherwise they would be sorcerers. or use CHA for spellcasting.
• the only option I can see being left is that some random Tree or forest or beach that is not a fey or god or any creature thus catalogued by the monster manual, is giving these guys the power to turn into badgers.

So I pose the question where do they get their powers from?

Edit.

It's not lack of belief that is the issue the struggle is we are doing a game where he is Dming and neither of us understands how it happens. which is a problem because he wants to create a game where we start at Level 0 and become our classes through the usual process. but we don't understand the process. so Will a magic tree give me the powers, will a trainer if a trainer does what will he say. will it be a nature spirit what?