Can’t recommend making your own harness enough. Even a trivial toy harness will clarify a lot of concepts that I see people confuse all the time (especially here on HN).

I made a simple video explaining how to make a simple harness in JS running in a browser, having it build it self up from running purely in the browser console to being interactive in the page to making downloadable copies of itself. Some people have found it super helpful.

https://m.youtube.com/watch?v=_AgKuFGvJfI

I would only caution that as soon as you’ve done this, you’ll start noticing a lot more issues in the OEM harnesses.