Idea on how to blend data from separate collections into one list

Ok, I have been brainstorming on this issue about trying to merge and organize data from two separate collections into one list. I think I need to start with this:

Is there a way to limit the results of a data variable to, say, for example, a list of doc ids or user ids contained in a separate data variable?

My current use case is like this.

I have a page to show all the comments on a post, with page parameter postId, which I use to limit the results of a get record collection to data variable function (built-in). I also order them by date.

Now, is there a way I can limit the results of a built in get record collection “users” to retrun only the users who’s id is included in the comment collection?

I think if I start here, it will be a good start in then trying to create a custom list using the data from both collections.

I do not want to write the users name and avatar to the comment collection, because i know from experience users get a bad experience if they change their user name or avatar and they do not see it update in their old comments.

I would really appreciate any help in figuring this out, as this concept need to be accomplished in many other ways.

If we can start with the first part, I think it will be key to making this work. What is the best formula to use to limit the results of a get record collection to only the ids contained in the result of another collection set as a data variable?

This seems to be something that would usually be taken care of by your backend. How do you have this setup?