It's just everything and the kitchen sink, I'm afraid, from a NATS server and client library over cross-platform GUIs including mobile, common file format reading and writing like Excel, Word, and PDF to post-quantum cryptography. I'm unfortunately in the business of Lego-brick software development using well-maintained 3rd-party libraries whenever possible. It's the same with CommonLisp, I love it but in the end I'd have to write too many things on my own to be productive in it.

I envy people who can write foundational, self-contained software. It's so elegant.