Are there any "Whiteboard" apps, programs, or services that will cross-platform between an iPhone and Linux?

My wife is looking for an app that helps her save information from brainstorming, and is visually helpful. Most times, she's using her phone to search out her ideas and save them (pdf's, links, text, pictures) on her iphone's "Freeform app", but wants to be able to open it on our computer for more "in-depth refining"

I'm looking at maybe "Miro" but don't know anything about it. Wondering if anyone has any other suggestions or more elegant solutions

A notebook and pen is always a thing she uses, but it's not as streamlined. I appreciate any ideas, or if there's a better community to ask this in