Indian passport holder traveling from USA to India and layover at Shanghai for 21 hour

24 hours only because of your passport. If you have hotel usually you can leave airport.

To increase (or even better to say, to secure) your chances to be allowed to leave the airport, book your hotel in advance and show the confirmation at the relevant counter.

This policy is firm. For T1 in Pudong (which is for those on all-China Eastern flights), no hotel reservation, no entry, no matter how long the layover is. Ironically, for T2 they still haven't build the international-to-international transit facility, so you will be forced to leave the sterile international area even if you don't want to and no matter how long the layover is.