This is a great lecture (in my opinion) that John Conway gave in Toronto in 2016 on how he came to discover Surreal Numbers. With that in mind be aware that he doesn’t do proofs early on, he states assumptions which he deals with later.
I’ve not written anything in a couple of weeks here because I started to write about infinity and I’m still unsure what I want to say about it. There is so much you could say about it.