Oh, I get it now, thank you so much!

Thank you for the responses, in fact I'm implementing a hardware wallet and I do have the mnemonic/seed. Let me explain further though.
Say you have a mnemonic phrase and a passphrase, you put them in a function and you get out a seed, you put the seed into a hmac sha512 function and you get the master private key.
The BIP84 derivation path is derived as such: 84th hardened key, 0th hardened key, 0th hardened key, 0th normal key for receiving addresses.
Now I can generate receiving addresses without exposing any private keys and my question is how do you spend those coins that are derived from the xpub given that you have both the extended master key and the extended account private key.
Thank you!

My apologies for the derivation path, how would somebody go about finding the corresponding private key to a public key that was derived from an extended public key?

Hello, since I want to learn more about bitcoin and how you would ago about a wallet, I'm stuck with being able to spend coins to an address derived from an extended public key ( non hardened) since you don't have the private key to do so unlike hardened keys. Say I use the BIP84 implementation, thus the derivation path being m/84'/0'/0'/0/0 for the first receiving address how do I get hold of the private key of said public key to allow me to sign transactions?
Thank you in advance!