Boston to Shenzen (have to arrive SZ on 12Dec2019)

There are no flights from Shenzhen to Hong Kong, and I doubt there is a Boston to Shenzhen flight either. If you don't have a visa then you must arrange your itinerary to include a third country, eg USA-China-Hong Kong or Korea-China-USA etc. The important thing is that you must fly Country A - one stop in China - Country B, and only the flights immediately before and immediately after China are relevant. Hong Kong, Macao and Taiwan count as third countries. For example, you could book New York-Guangzhou-Hong Kong-New York flights (which would qualify for 144 hours transit without visa in Guangdong) and take the high speed rail to Shenzhen. There are also sea options from Shenzhen (slightly more complicated - see the 144 hours visa-free transit in Guangdong page).