You are very lucky. In China, virtually all websites are required by law to use your phone number (verified by SMS) to register and/or to use. And all numbers must be linked to your ID.

What do you do when the car repairman wants to call/text you when the car is finished? Or any other similar situations?

No car.

But in general, email.

I can make calls from my phone/laptop, using VOIP.

I could receive as well if I wanted to, but I rarely need to be called, so I do not normally keep a number, and I could not be called when out and about anyway, because wifi-only, but you do get an answerphone, so people can leave a message.

I use VOIP numbers for that. One per business.

How do you do online banking?

Laptop, running over Tor.

I use cash for all physical payments; never the card. I use the card to withdraw cash, once a month.

I very rarely buy on-line, because I stopped using Amazon (treatment of warehouse staff) and buying off of Amazon is hell on toast.

Given the rampant privacy abuse on phones, I use a laptop.