Hi Mike, try this:
const threads = await wixData.query("threads")
//take out the _id value of the first item in the array
//and use it to find the threads that belong to that forum
.eq("forumId", forums.items[0]._id)
.find();
note that in as far as what is stored in a collection for a reference field, we only store the _id of the target object.
so you need to use that _id value in your query filter.
hope this helps and let me know if you need more info on how references work in wixcode.