Modern smartphones, iPhones, Androids, Samsung, etc. are more computer than cellular phone. The cell phone companies don't make this clear because most of them want you buying phones from them on a finance plan to increase their profits and they want you dependent on them. Apple charted a new way when they created the iPhone: you could by the iPhone from Apple and then activate it for phone service from a cellular provider. Most Androids and Samsungs seem to be purchased from a cellular company or from a retail store that sells them with cellular carrier. You can buy Androids and Samsungs "unlocked" and ready to be activated on any carrier.
If you have an Android, you get a Google or GMail account. If you have a Samsung, you may have a Google account and a Samsung account (that's why I am listing Samsung separately from Android). If you have an iPhone, you have an Apple iCloud account. You also have an account with the carrier and a phone number with phone service provided by the carrier. There are some cross connections: Apple insists that you use a phone number to create an iCloud account. Google wants your phone number for password resets and the like. But in spite of these cross connections, your handheld computer that you call a phone has two completely different "accounts": the cloud account for the phone from Google, Apple, Samsung, etc. AND the phone service account from the carrier.
Your "phone" doesn't even have to have the phone service and phone number. You can set up a smartphone like a tablet using only WiFi with only your Google or iCloud account. Tablets are really just larger smartphones. Some have cellular hardware in them, some do not. Looking the other way, a phone is just a small tablet that always has cellular hardware in it.
This has implications…
If you are upgrading to a new phone, you can still use the old phone if you have a WiFi connection. You can't make phone calls, but you can do anything else you can do on the phone using the WiFi connection to the Internet.
If your cellular contract is up so your phone can be "unlocked" or you have an unlocked phone, you can switch to a new cellular service provider without changing your Google or iCloud account. Thanks to FCC rules that encourage competition, you can "port" your existing phone number to your new cellular service provider or you can get a new phone number.
You can also lose access to your Google or iCloud account but still be able to make phone calls and get messages.
Be aware of the cross connections, like the phone number being used for 2FA or password reset.