Sadly I'm not all that clear in the head and I'd be making assumptions based off that and need a more layman-friendly answer... would one private key of one address, and a bunch of public addresses jeopardize the entire wallet's private keys?

ALSO, if that key was P2PKH and I only use P2SH-P2WPKH keys, does it extend to them, or would it only compromise that one branch?