So what happens? If there's not a backup phone number or email set, what could they possibly ask for beyond the password?
They ask for your current mobile number -- at least in germany and without any anonymization beside standard browser configuration like e.g. uBlock Origin.
So it's simply to suck in numbers.
I have not tried if they ask for more data after giving a phone number and so I can't log into two google/youtube test accounts any more since a few years.
The phone number is the last step, at least it was for me. I had luck with virtual phone numbers, not the freely available ones, but paid apps like e.g. Vyke.
So what happens after you use Chrome and type in your number?