Of course you can use it, don't be silly! The government doesn't actively monitor your apps or internet activity.

Or does the operating system or ‘WeChat’ immediately report it to the government?

As if the glorious bastion of freedom and democracy, La Estados Unidos doesn't report anything the user does to the government.

The government is pretty lax in terms of what apps you use and if you use a VPN

I mean if the goverment can see which apps you have installed (can they?) then they probably wouldnt like briar. Briar itself is a very good and secure project, that can still work even if the goverment shuts down internet access. So if you have the option of putting a custom operating system on your phone, then briar is a good solution for anonymous communication.