I found the perfect system for me and I stick to it. I use a folder of Markdown text notes organized by creation date, with a text editor (ideally something like nvAlt / Uulysses / Obsidian that does indexed searches).
Titles are:
2022-12-26 Note Name.txt
That way I can find what I’m looking for, usually instantly, even though I have a pile of thousands of notes in that folder spanning a decade.
I haven't found an iOS app that allows good search in plain text files.
About every app out there uses Apples standard search control, which finds files where text occurs. Not the line(s) with the occurrence itself, no forward/previous, only a list of files that contains the text.
Maybe you'll like 1Writer for iOS. It lists files including the line you search for, and it supports the "YYYY-MM-DD Note title" syntax mentioned in the commment above. So you can point the app to the folder where you store your plaintext files.
I like Zettelkasten / bunch-of-markdown-files tools because more often than not they _expand_ on this simple idea, offering additional structure on top without aiming to destruct anything.
Titles are:
2022-12-26 Note Name.txt
That way I can find what I’m looking for, usually instantly, even though I have a pile of thousands of notes in that folder spanning a decade.