Getting user data by ID (Facebook API)?

Question

PHP

Good afternoon.

We make applications for integrating facebook pages with CRM.

When we receive a WebHook for some event on the page, we want to receive minimal data about the user.

There is one suitable event "feed" and maybe "members" (but it is not clear from the latter what data is coming).

So when we get a new message for "feed", will we have data about the user himself?
At least its ID? or there is only the id of the message to which you can reply.
In the test notification, there seems to be no data about the user Id :(

And the second main question, if we still somehow get the user Id, can we get the minimum user data through the API?
In the "Graph API Explorer" it is not possible to get data for any user Id .
I met somewhere that the application will have access to users who have installed the application.

There is a possible reason that our application has not passed the test yet, but is there any point in doing it at all if no data can be obtained ...

And of course, there is a suspicion that they have covered all this since April 2018.

PS We also want to do something similar for Instagram, but they have our own events, for which if we get a user ID, we would also like to save minimal data. If you know how to do it here, it will also be relevant.