MARYSUE | Novel generator written for NaNoGenMo | Animation library

I am working on my push notifications to users and working through the documentation.

1. Following this walk through, I have set up a webhook to send a notification to a user, this notes a target.userId for the recipient. Question, what exact value is the referring to? Is this the userId I invent for a user, or is it something else?
2. Looking at userId documentation, it appears this has to be generated/maintained, and can go stale every 30 days of inactivity. Noted here

My concern is - users sending other users notifications. If the recipients userId has gone stale from what I've stored in my database, I assume the push notification will fail when trying to find that user by userId that expired? Is this the only way to target a user for a push notification? My assumption is yes based on this documentation - but it seems brittle - or unstable, since it rely's on an ID that can expire (unless i'm misunderstanding userId's).

Is what I've outlined above the correct way to think about push notifications from one user to another? I store the users ID in my database, and when a message is triggered ie: "tell JimBob that I'll be there at 11." i look through my DB to find JimBob and his saved userId, and then send JimBob the push Notification from MarySue.

I assume this also denotes that JimBob has to allow push notifications from the intent "receive_message".