This is not on the topic of the stack overflow question. **This is on the topic of my specific problem.**

Failed-Attempt-1.py

# Check comments to see mypy/pylance complaints

# Failed proof-of-concept code

from typing import Type
from dataclasses import dataclass
from dataclasses import field

class A:
    def __init__(self, args: 'BBuilderArgs'):
        self.a1: int = args.a1
        self.a2: str = args.a2

@dataclass
class BBuilderArgs:
    a1: int
    a2: str = field(init=False, default="this is some parameter I don't want to change while initializing")

class BBuilder:
    def __init__(self, args: BBuilderArgs):

        class B(A):
            def __init__(self):
                super().__init__(args=args)

            def method_that_A_does_not_have(self) -> None: ...

        self.B: Type[B] = B

    def new_b_instance(self) -> B: # "B" is not defined   <----------
        return self.B()
    
my_b_args: BBuilderArgs = BBuilderArgs(
    a1 = 123
)

my_b_builder: BBuilder = BBuilder(args=my_b_args)

my_b: my_b_builder.B = my_b_builder.new_b_instance() # Variable not allowed in type expression     <----------
    
# I guess making a Class Builder (this is probably not a real term) in a way that satisfies mypy and pylance is way harder than I thought