How do I factory reset a used iPhone without knowing the passcode?

You can follow the instructions here to get past the passcode: If you forgot your iPhone passcode - Apple Support

However… if it is activation locked, there is no way to bypass that. Only the owner of the iCloud account that was used on it can remove the activation lock. If they can’t or won’t, it’s a brick.

Without the Apple ID and Password of the original owner, you will never be able to use the device. You will encounter Activation Lock which is a theft prevention feature. We cannot help and Apple will not. Please read this link...

https://discussions.apple.com/docs/DOC-250001038

You can recycle the device at any Apple Store.