Pixelmator Photo, iOS only and previously iPad only, is now available in an iPhone version. The introductory sale price is $3.99. If you already own Pixelmator Photo for iPad, you can get it for free. (You have to tap the $3.99 buy button, but then it'll figure out you have the iPad version and note that it's free.)
I've only taken a brief look at it so far, but it has all or almost all the tools that the iPad version does. The layout is different (because iPhone vs. iPad), so some things take more taps to get to. It does have the excellent resizing (up and down) of the iPad version, which I use frequently. The most glaring omission from both versions is layering, which is odd because the older app -- plain Pixelmator -- does have layers even though it has fewer features.
Two further notes on Pixelmator Photo:
* It offers a variety of auto-adjustments labeled ML (for machine learning). These generally produce decent results but often a bit too light for my taste. I seem to prefer a slightly darker look. But they work quickly, so they're generally worth a try.
* Its SuperResolution function for upsizing images is the best I've found in iOS apps. The quirk in the upsizing is that the app doesn't give you a choice of size -- it determines the best size it can create on its own and does that. For downsizing, which is also very good, you can set the size as you'd expect.
On the whole, it's a good app, not as powerful as Affinity Photo but more nimble. If/when it adds layering and masking, it'll be a really good app.
And just to avoid confusion in naming:
Pixelmator = older, less full-featured app for iPad but with layers
Pixelmator Photo = flagship apps for iPad and iPhone but without layers
Pixelmator Pro = desktop version for MacOS only