Immigration Rules for Hong Kong

If you have valid pre-arrival registration to HK, you can stay in Hk for less than 14 days without visa. If you don't have the HK pre-arrival registration, you will be fine if you don't leave the transit area to enter HK for sightseeing during the layover time. I suppose that you may need to go through both the immigration process in HK and Shanghai.

There's no need (and you can't) go through immigration in HK without a valid pre-arrival registration confirmation. If you attempt to do so you'll be in serious trouble. Just follow the "transit" sign.