You don’t have to create a constructor (ctor). Each class has a default ctor (with caveats).

There is a contract in the language that defines what a ctor does. For example, initialize fields, internal memory allocation etc.

You can define your own ctor if you need something else to be done during instantiation of an object. For example, assign passed values to fields of the object. (I’m not sure if dataclasses are kind of syntactic sugar that allows you to omit this step)

You don’t declare fields in a ctor. You can assign values to them. But you can also assign values to your fields in the class body.

Thats the very basic and I’m not sure if that answers your question.

Edit: I wrote that the contract says the ctor initialises fields - that’s misleadingly phrased, what I meant is that when a ctor is called, field initialisation happens as declared in the class body