Im using web3.py 6.0.0 with code:

import os
import json
from pathlib import Path
from web3 import Web3
import time
from dotenv import load_dotenv

load_dotenv()

class SmartContractInterface:
    def __init__(self, nft_contract_address: str, owner_address: str, http_provider: str, type_of_nft: str):
        self.w3Provider = Web3(Web3.HTTPProvider(http_provider))
        self.nft_contract_address = nft_contract_address
        self.owner_address = owner_address
        self.base_dir = Path(__file__).resolve().parent.parent.parent
        self.abi_path = os.path.join(self.base_dir, f"backend/utils/contract_abi/{type_of_nft}.json")
        with open(
            self.abi_path            
        ) as f:
            nftAbiJson = json.load(f)
            self.nftAbi = nftAbiJson["abi"]    
        self.nft_contract = self.w3Provider.eth.contract(address=self.nft_contract_address, abi=self.nftAbi)        
    
    def transferNftFromCustodialWallet(self, user_address: str, number_of_nft: str) -> str:
        """ Transfer NFT from custodial wallet to user Ethereum address"""
        try:
            event_filter = self.nft_contract.events["Transfer"].create_filter(
                fromBlock="latest"
            )

            ###### THIS FEW LINES ARE RELEVENT FOR THIS QUESTION ########
            self.nft_contract.functions.transferFrom(self.owner_address, user_address,int(number_of_nft)).transact(
                {"from": self.owner_address}
            )
            ###########################################################

            events = event_filter.get_new_entries()
            for event in events:
                if event["event"] == "Transfer":
                    return (True, event) 
                else:
                    return (False)
        except Exception as e:
            return e

Where for owner address I use eth address of NFT token owner who have rights to make this transfer.

But now I'm puzzled how web3.py know which private keys he should use to sign this transaction just by knowing public address? From where does he gets this private keys?

It looks like some web3.py magic is involved under the hood? I don`t have other explanation...Some ideas?