I can tell you that submitting works to the Library of Congress isn't as straightforward as it might seem. The Library of Congress primarily acquires materials through copyright deposits, purchases, gifts, and exchanges. If you're an author, one way to get your work into the Library is by registering it with the U.S. Copyright Office, which then sends a copy to the Library. This is a common method for many authors.
However, the Library also has specific collection policies, and not every submitted work is automatically added. They focus on materials that have historical, cultural, or research value. For self-published or indie authors, it might be more challenging, but it's not impossible. I’ve seen many niche works find their way into the Library through persistent efforts or by being part of a larger collection donation. It’s worth noting that the Library doesn’t accept unsolicited submissions directly from individuals unless they align with their current collecting goals.