Activation lock

You will need to contact the person whose account the phone is activated to. They will need to remove the lock. If you cannot do that you will never be able to use the phone. So first of all you need to go back to whoever you bought the phone from.

Hello there. Has the seller provided you with the original receipt? If so, you can contact Apple and get them to remove the activation lock if you have a valid receipt. (Remember to make sure it has [at least] the serial number). You’ll need to visit your nearest Apple Store and give them your proof of purchase. Find your nearest Apple Store: https://www.apple.com/retail/

Hope this helps :)

I think that only works for the original purchase. A receipt for a second hand private sale (or from a company so unreputable it is not possible to contact them) is not likely to be recognised by Apple.

No, not really. If you cannot get hold of them there’s nothing you can do.

Hopefully you bought through a platform that has buyer protection - eg eaby, or amazon etc. Without the original apple ID, the phone cannot be used, and needs to be returned for a refund.