After reading through the expo notifications api, I still have no idea on how to implement push notifications to selected users on different devices dynamically. For instance, after a user made an activity booking in the app on behalf of a few users, all the users involved will be notified of the activity one day in advance. My database used is firebase which includes the participants involved in the activity.
First thing to note is that you must use the Expo Push Notifications API if you want to send out notifications. No OneSignal etc.
To answer your question, though, Expo have a doc article about this: Notifications - Expo Documentation