The vast majority of people keep the same phone number for a long period of time; at least a few years in most cases, and several decades in some cases.

The telephony system relies on that being the case, which is why it has the nice byproduct of being an identity verification channel for banks and other service providers.

So it doesn't matter who strictly 'owns' the phone number. What matters is that it can be relied on to be linked to a given person over an extended period of time, which it usually is, which is why the system usually works. And when it doesn't work, a fallback process is available, which makes the system work even better. So all's good!

It's easy to point out ways in which the system fails in all kinds of scenarios where people don't/can't fit in with the way the system is designed, but it's ultimately futile.

Service providers will run their system in whatever way is secure enough and usable enough for enough of their customers to be satisfied to stay as customers. There's really nothing more to say about it!

> multiple people using the same google accounts

Why would you do this? Whether personal or work related, Google has always said to keep accounts tied to an individual.

Team managing a GA account for a client. Which is not uncommon for internet marketing companies.

exactly any non trivial site that uses google products will have accounts used by multiple people one person doesn't run all of proctor and gambles PPC

I keep multiple gmail for each purpose.