How do I migrate Apple ID to a new email?

So, I got my old iPhone as secondhand from a relative, their Apple ID was still logged into it when they gave it to me and they never used it afterward because they changed their phone to an android. I've been using that Apple ID for a couple years now without care much because I was the only one logged into it and there wasn't an issue but now that I have a new iPhone, I want the Apple ID of my devices to be my own. The new phone I got was secondhand as well but this time it was factory reset to the Hey, Hola, Hello screen or whatever that's called when you get a new iPhone. I forgot the process I did to transfer everything from my old phone (including iCloud/Apple ID) to my new phone but I remember I scanned something on my new phone using my old phone and I guess it was some sort of QR code but for Apple devices. Now I'm looking up how to transfer/change Apple ID email to my current Gmail that I use on a daily basis(Basically my go to email when I need to sign in to something). Attached is the email & phone numbers connected to my iCloud with the Gmail being the said email I want to change the old iCloud into.

Trying to remove the iCloud email via the little i button next to it, it says this (Image attached below). But when I check iMessage and FaceTime settings, everything is turned off for the iCloud email.

Going to the Mail section of the settings app then going to accounts, the iCloud is still the account receiving iCloud things and not the Gmail. Blurred are emails that don't concern this matter)

Going to the icloud.com website, I can login via the Gmail email and the iCloud email because for some reason the iCloud email is an "alias" now. As you can see, the Gmail is now the Apple ID but I think it's only because I tried replacing the iCloud email to that email.

Going to Mail in iCloud.com then going to settings then accounts, the primary account is the old iCloud one.

Apologies if this post is rather long, I just wanted to show everything I've tried and done so far. All I want is for the old Apple ID that belonged to the relative that gave me my old iPhone to be replaced by an email that is under my name and if I can do so using my current Gmail that I wanted to use in the first place.

I don't want any data lost from the old Apple ID

Big mistake here was not creating my own Apple ID when I got my old iPhone, I see that now... I see it clearly.. Thanks for the help and feedback