Well, bell training is basically just a way to give a dog the means to let you know that they need (or want) out. Not every dog (or even most) actually thinks of barking or scratching at the door

But all can be taught to ring a bell when they want the door to be opened.
What it
isn't is actual housetraining. It doesn't teach the dog that they should potty outdoors. It is purely a means of giving you an audible signal that they require the door opened. Once the dog is properly housetrained by normal methods though, then the bell training comes into it's own as a means for them to let you know that they need out to potty. Up until then, it's just a means of letting you know they'd like you to open the door (for whatever reason).
And it's reasonably simple to teach. You just need to hang a bell - or any other thing that will make a noise loud enough for
you to hear and that you'll recognise - beside the door. You then teach the dog to hit said item with it's paw or nose prior to you opening the door. You'd have to do that every time you're going to open the door for the dog, whatever the reason. And you'd also need to respond any time the dog rings the bell - for whatever reason (including, initially, when he only wants out to play). Once he has it properly set in his mind that hitting this object will result in you appearing to open the door, you're well on the way to a bell trained dog and can start to be a bit more discriminating about when you answer (ie. can start to ignore the requests to go out to play).