I take my notes in INI format with a lose schema, as I accumulate data I tend to move towards something more concrete and write tools for it. I think this is the absolute best compromise between some kind of formal personal ERP-like (PRP?) system and something super loose like Markdown or org mode.

Of course doing this on an iPhone is an absolute nightmare because everything has to be blessed by Apple and you can't just do one-off ad-hoc automations or usefully compose tooling that touches the filesystem. Everything has to be canned and sharecropped (at best) so them adding Markdown to the only text editor that supports fast, energy efficient background sync is a huge deal.

When I had an iPhone I did try doing some server-side automation with the SGML-like (can't remember if it was actual HTML or not) format notes used. Like most of those sorts of things it was a miserable uphill fight to get value out of the thing. I've been so happy ever since I've completely given up on anything smartphone related.