Similar experience, I helped to build the website, manage the forum installation, and overtime the people running the shard invited me to also code for the emulator they were running.
I was 12-13 at the time (late 90s/early 2000s), I can't remember the emulator anymore, very likely it was POL, and the concept behind the shard was to be as close as possible to the official servers before UO:Renaissance so we worked quite a lot to make it look and feel as T2A. I learnt a lot, later when RunUO came out and became a bit stable (circa 2003) I helped to migrate what we had done within POL to C# code for RunUO, had to learn a lot more to keep up.
The other people working on this shard were all in university studying CS, or already had jobs as programmers, I was the kid who could write some scripts, I believe having this experience was pivotal for me to later become a professional. My first job in a real tech company even came from a recommendation one of these guys made when an internship position opened.
In a way I probably only have my career because of UO and private shards.
Same, when I got into UO emulation I used Sphere for a month then this new C# emulator called RunUO was announced haha and I moved over (~ Aug 2001). There's still a forum post from 26 years ago of me remarking about all the brackets { } involved in writing C# code ha. But I remember that spark when I made a change in a sword file and saw it reflected in game. Been chasing and riding that high ever since. Led to a career in software development but more importantly unlocked some of the greatest joy in life I'm sure everyone here can attest to.