I used to love Yojimbo. But alas, I want something across macOS, iOS, and iPadOS. I use DEVONthink for a very specific purpose. And I know they have an iOS app. I should explore that again for this purpose.
My use for DT is this: I am a researcher/writer. I keep all research references and their associated PDFs in Bookends (and I write primarily in Mellel, which integrates beautifully with Bookends). The Bookends PDFs are indexed by DT. This way, using DT I can search for a specific word/phrase across all the PDFs in my Bookends library, see the results in context, etc. Very useful when writing.