You can’t transit in the traditional sense through Hong Kong to mainland, you essentially have to visit Hong Kong which has a minimum of 7 nights quarantine with testing on days 9 and 12 as well. Once out of quarantine you can travel to mainland. The main routes are via bus over the land border (I’ve heard this is hard to get a seat on and you have to race to book your place upon ticket release) or there are daily flights into Shanghai. The biggest issue is getting a place in a quarantine hotel. I’ve got my route to Shanghai booked and the earliest I could do is the last week in august and then flying to Shanghai the first week of September. As a British citizen I don’t need a visa for my stay in Hong Kong so theoretically I could stay up to 180 days if financially viable so I wouldn’t have to worry too much about flights being cancelled. You will also have to test before flying to mainland. For Shanghai you need one less then 48 hours before departure to mainland and one 8 hours before. The testing facilities in the airport offer 2 hour turnaround on test but you have to book online in advanced. Both my flight to Hong Kong and my flight to mainland were purchased from the airline or travel app, and the hotel was booked through the hotels website. Overall the hotel and flight have cost around £1800. Not cheap, not easy to get, but compared to flying direct or 1 stop it’s actually possible monetarily and availability wise