Another story from my youth: I was about 7 years old. My father was friends with this Egyptian gentleman who was completely deaf and was never taught how to speak, nor read, nor write, completely nonverbal and only signed. He was one of the most talkative and opinionated deaf people I think I've ever met. I'd see the man get into political arguments while signing. What was interesting was that my father knew how to speak to him intuitively. It wasn't any standardized sign language, and yet he knew it fluently.
And another story about a deaf person: my cousin on my dad's side who's also deaf (though able to write and speak to some extent), works as an animator (and was on some pretty big productions!) and draws some of the best art I think I've ever seen. Now, she and her sisters were quite close to my father, who spent summers in Germany with them. When we spoke, I used my notes app to write things out so she'd read them. I didn't know how to sign (and still really don't). Anyway, they had specific signs referring to specific people or things; for example, when they wanted to talk about my father, they would open up their palms and curve their fingertips inwards, like a claw hand, since my father would often do that (and still does).
What's fascinated me is the expressiveness of these homesign systems. Since both are adults over 30 (at least of the writing of this), their signs can be quite sophisticated. The fact they can sustain complex conversations on politics through pure signing alone and reproducively to boot in that other people can use the same exact system is remarkable. One interesting aspect was this: the Egyptian man's signing was evidently in some part inspired by Arabic word order.
When he wanted to talk about the president of Egypt, Abdel Fattah el-Sisi, he'd first sign "president" (putting fingers on shoulder, as if to signify a military rank), and then "Egypt (my home)" (putting his two hands above his head to signal a house with a roof), before the predicate. On the other hand, in English, the possessor generally precedes the possessed. (though the construction with "of" works too, so "the president of my country"); this could be of course be down to his conversations with people who do speak Arabic natively (so, pretty much all of us).
Looking back, this was one of the first times I think I was confronted with a theory of language. On one hand, the system was evidently spontaneous and somewhat rudimentary at least in comparison to a standardized sign language. It developed as the guy's need for communication did, and with the people he communicated with. But on the other hand, I wondered how that many minds could come together in developing a manner of communication purely through gesturing, and I couldn't understand them, if it's completely and utterly built as a hodgepodge of different people's manners of signing? This is where I was first confronted with the formality and reproducibility of language, and the spontaneity and functional, pragmatic aspect.
And this, to me, is what makes language construction (both as in homesigns as in conlangs) especially interesting. The fact that you could engineer a language in accordance with syntactical and morphological constraints while allowing it to be used expressively still blows my mind. In some manner, language is as much an art as it is a science as it is engineering. An art, in that you could encode a philosophy in a language (see Toki Pona or Lojban), a science in that it is simultaneously a biological and cultural force that you can analyze acoustically, biomechanically, cognitively and socially, and engineering in that it's a modular system of rules, constructions, whichever you prefer, one that can be built and developed over time.