The last time I updated my resume, I took a bit of a different approach to formatting it that I think worked really well. Instead of the standard sections, I wrote it in prose where I basically list each job and wrote a couple of short paragraphs describing what I worked on and things like if/when I was promoted (as in some companies I had multiple titles as I was promoted up). Having been on the hiring-people side more often than the getting-hired side, I find it much easier to read than a big list of bullet points and a (IMO useless) "list of technologies I know" section.
To answer your question of where I put it, yes I put it as another "job" in my experience section. So after my startup folded, I was working on one of my personal projects (an iOS app that's in the App Store that I previously worked on as my job as an indie developer and had picked back up). In this case I modernized it to properly support the latest iOS versions and devices and started porting it from Objective-C to Swift.
This worked out great as I had essentially no gap in my resume since I started working on it again right after I stopped working on the startup. So even though I wasn't getting paid, it showed I was working on something at least (and in this case, something on the appropriate level since I was rearchitecting the code).
Here's a part of that section as a reference:
I approached the project as a senior engineer cleaning up and modernizing code written years ago by a junior — who happened to be me. Within a few weeks I was able to fix all of the core issues preventing its use on iOS 14, modernize the UI, and remove all deprecated APIs (some going all the way back to iOS 3).
In addition, I've been working on a major re-engineering of the data model, porting the entire app from outdated Objective-C to modern Swift 5, and beginning to add new user facing features allowed by the new data model.
I also linked to the GitHub page so they could see the work I did.
As for your question about when a project crosses into resume territory, I would argue any non-trivial project is fine. Even if it's a new project you start from scratch, I think as long as it's the type of work your would be doing if you were hired (in my case I was doing architecture work, etc) or even if say I had started a non-trivial project in a new language I didn't know just to learn it (let's say I started a non-trivial Rust project or something) I think it would also be fine. And I don't think it matters if you've finished it, in progress work is fine as long as it's on Github so they can review it. It's really up to you, but I think the point is just to show you continued to work at your "level" and/or continued to learn it's fine.
This is all just my opinion both from having used it successfully in my own resume and from having been part of the hiring process of many developers, but I can at least say I didn't get any negative feedback about it and I did get the job. This was with a full 6 month "gap" after ending my last position.
My Github username is the same as this username so you can see my projects there if you want (the one I'm referencing here is called iSub), and I'm happy to DM you my resume if you want to see it as it contains personal information I'd rather not share publicly like my phone number (though I guess I could redact it and just post a link, this profile isn't exactly meant to be private or I would have used a different username).
Hope that helps!
Project lead (or maybe one of them I’m not sure) just left too: https://stgraber.org/2023/07/10/time-to-move-on/
Ouch lol