I suspect both Russia and China were operating different model balloons for different mission types.
What are your guesses on their purpose?
If I had to guess, I wonder if they are data relay nodes to collect information from Chinese/Russian bugged equipment that cannot transmit over the internet (eg. compromised equipment in secure military facilities).
An interesting theory, although I would be at least a little worried if secure facilities did not already have some form detection of signals they do not expect.
OTOH, you could instead have something gathering data, and the passing balloon could send a signal to the listening device(s), and then quickly dump+relay the data collected over time.
A lot of the comments focus on "Why would they use balloons when they have satellites?" But the reality is they are cheaper, can be oriented inexpensively and can loiter. Not to mention the lower radar signal and other detection issues (made clear by the revelation that this has been going on for a while).
Now that launching/steering balloons is down to the hobbyist level...it seems a larger corp or state actor could easily use them in a way that seemed impossible 30 years ago.