I sold my first novel (a western) in 2019. Since then I have written a sequel. Quick question: How much of the first book should I reference in the query for the sequel (e.g., should I mention it at the beginning in terms of where the sequel picks up the story line)?
That's not the question you're going to run aground on.
It doesn't matter what the first book is about.
It matters only that there is a first book.
Most publishers don't want to acquire Book #2 in a series if they don't have rights to Book #1.
Why is that you might wonder.
It's because a decent percentage of readers like to read series in order.
Which means all the promotion Publisher of Book #2 does benefits not them but Publisher of Book #1.
There are exceptions to this of course.
Generally an author is able to move a series if Publisher Next thinks a wheelbarrow full of money is to be made.
But to answer your question: you don't mention any of the plot of Book #1 in a query for Book #2. You say only that it's the second book in the series.