One of my best friends visited the Mainland for the first time in five years last month, and getting a new SIM card was an agenda item. He wanted pre-paid, but I'm not sure those exist any more, and I'm also not sure if it really matters (i.e. just get the cheapest post-paid plan on offer and add data/top up, as needed via WeChat or Alipay. Suffice it to say, another friend of ours brought him to a China Mobile store and they tried to sell him a really dumb plan (I would have taken him a Unicom store instead because I know they are cheaper), and we concluded over chat that he really wouldn't derive much value from having a local SIM because he was already set up on Wx/Ap, daily roaming charges on TMobile weren't egregious, and he had no problem receiving voice calls from Didi drivers (i.e. they don't dial foreign numbers directly; the system puts them through).

So, in his case, TMO proved to be an okay enough solution for 10 days in Shanghai and neighboring cities. He complained to me several times that TMO's data was slow, but it's not like he was attempting to do Zoom calls during subway rides.

That having been said, getting a local SIM certainly isn't a bad thing, as long as you don't allow yourself get scammed by middlemen (airport kiosk people usually qualify as such) or lock yourself into a plan that doesn't work for you.