I am unable to log out of someone else's Apple Account on my iPhone 15 Plus

Check the requirements and steps at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” Apple may typically require the original receipt* issued by Apple or an Apple Authorized Reseller. If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support This is the receipt from Apple provided to the original purchaser who should have given it to you.

If you cannot get Apple to unlock the device, return it to the person from whom your purchased it and ask for a refund if they cannot unlock it with their account information.

Sorry, but there is nothing anyone here can do for you. This is a user forum.

If Apple says the documentation you have is insufficient, then there is no way to remove the activation lock.

Contact the owner of the account that’s logged in on your phone. How, exactly did that happen anyway?

This. I’ve never heard of Apple requiring someone produce the original product packaging.

*shrug* Apple China?