The three biggest wireless carriers in China are China Mobile, China Unicom, and China Telecom. The first two are GSM carriers while the latter is a CDMA carrier.
s fI was about to say something similar. China Unicom is fantastic in the city and I feel has faster download speeds than China Mobile, but overall less coverage when you get out of town. I’ve found the best overall coverage in the countryside (I frequently travel to rural Yunnan) is China Telecom. Best advice if you do travel within China a bit, get a dual sim phone (iPhone X R has this). I get a China telecom SIM card includes with my internet, so I get the best of both worlds
1/ you can’t port numbers (ie transfer your number from one carrier to another) like how we do in the states (ex. keep your number from AT&T when switching to Verizon in USA)
2/ you can top up a number of ways: in store (bring your passport if foreigner), their app, or simply via wechat. Go to your wallet page and look for “Mobile Top Up”. You can do this in Alipay as well i believe
3/ China Mobile has been the golden standard for people who travel outside of urban areas, as their coverage seems to be better when going to more suburban and outside 1st tier urban centers like SH, BJ, etc