I'm very interested in how do you run the said network analysis? I'm considering buying an Android again but I'd like to properly tame it by cutting off as much spy traffic as possible.
I don't remember what I was using at the time, but there's a variety of ways to do it, depending on what you're looking for. Generally, you want to capture network traffic at the packet level and run it through a tool to help you analyze it. The tool you pick would depend on what you're looking for and the type of analysis you're doing. Here's a few tools that come to mind:
Not the original parent commenter but a sniffer like Wireshark would reveal the url(s) or IP addresses being requested by the phone to call home - it’s fairly easy to check if those belong to a Chinese IP range.
What you are looking at is like buying an iPhone and wanting to block Apple. You won't be able to from the phone itself (unless you flash another ROM but then you'd be better off buying something like a pine phone).
If you are really serious and you won't flash a new ROM then you will need to filter the traffic after it leaves the phone as you cannot trust the phone or its apps. IMO the only way to do this is to use a VPN to connect to a device that runs a proper firewall. It could either be a Raspberry Pi you carry around (bit too geeky for me) or a server somewhere you trust. I connect to a pfsense machine at home via VPN and filter traffic with pfblockerng. You can block on geography if you want.
My girlfriend's iPhone is just as bad as my OnePlus..
https://www.wireshark.org/
https://www.snort.org/
https://www.ntop.org/products/traffic-analysis/ntop/