Warranty registration how to do?

You don't register for warranty. Warranty starts the day the phone left Apple inventory. Go to Settings > General > About and look at the box beneath Serial Number, where whatever applies to your phone can be seen there.

Hi, Apple use to have a site where it would let you register and manage cases for all Apple branded products you purchased that came with a serial and warranty.

i see some of the responses and am a bit lost. Not all devices are purchased directly from Apple. Their are tons of resellers etc that purchase products. Products could be purchased by a person or business that doesn’t what to use icloud. There’s got to be a way to manager your products online when you don’t want to link them to any iCloud. And im talking about more then just phones. AirPods could be used with an android, apple’s pro monitor? Etc.

second question.

I purchased a 4 pack of air tags. I registered one to my icloud acct. it was showing up in find my app. Last night it suddenly started beeping and I looked at my phone and it alerted me that an air tag was following me when I looked in the find me app my air tag was not there my initial thoughts where that someone could’ve swapped out my air tag for someone else and then registered that air tag to another account that’s the reason why this air tag wasn’t showing. After getting notified I called Apple and asked why this air tag fell off my account and they told me they had no answer and to not worry about it and she just registered again. So I took another air tag from the four pack and I tried to register that one then I got a message that my encryption on my device my security on my device/encryption needs to be upgraded and it would take me too my iCloud account screen to manage but there was no options because I supposedly already have two factor authentication and that’s the highest level of offer authorization on this account. The representative could not explain why I was getting this message and basically had to play reset yeah turning off the phone and turning it back on blah blah blah until it finally registered without asking me for that my second question was could you tell me what the serial number was for the air tag that fell off my account because IWant to make sure that before I use my original air tag that it’s not someone else’s. I was then told that they wouldn’t know what the serial number is so they couldn’t verify if the air tag that I had was mine or not apparently there’s no history or no site that you could see the products you purchased even when they’re not linked to your iCloud account

second question. I purchased a 4 pack

You don't. There is no need. The warranty follows the device and is automatically activated at the time of sale.

You can check the status of your coverage here: https://checkcoverage.apple.com