> I see on the web now you get round the great firewall just by using a foreign esim

Yes, but that's not circumventing anything. That's an essentially approved method for foreigners specifically because they don't care about keeping foreigners within the GFW.

Once that foreign esim is behind a GFW equivalent of whichever country it is from, what then?

> I think places like China put in a lot of effort

Yes, which is why China was tricky.