Where is pin 1 on the DB820c?
Does all board extensions work with DB820c?
You can find hardware documentation here: https://www.96boards.org/documentation/consumer/dragonboard820c/hardware-docs/. All 96boards consumer edition have the same pinout, you can some info on https://www.96boards.org/pinout/ as well.
I would say yes, but this mainly depends on software(drivers…) support. If you have a specific one in mind, do not hesitate to ask on the forum before purchasing.