Fileshare witness for Multisite AvailabilityGroup setup
Best case - third independent site and/or online somewhere that you can reach it from on premises. Otherwise, you need to put it in one of the sites (and it also needs to be highly available). QUorum is a model. You're configuring a witness. General rule of thumb: The core cluster group and the witness should always be in your primary site since that's where you care more.

You may potentially need to adjust votes on the nodes, and both sides should be able to see that FSW. If you need to fail over to Site B, you'll need a new FSW.