What cool project can I do with an extra, barely used iPhone XR?

If its on a version you can jailbreak you now have the worlds most performant raspberry pi with all that hardware from the phone already hooked up.

Maybe you could start with just a full phone teardown? Follow the guide here - https://www.ifixit.com/Teardown/iPhone+XR+Teardown/114123.

The logic board is where the soldering iron might come in handy but i think components on modern electronics are too small and integrated for that to be of much use for a beginner. Plenty of other good ways to get into electronics & soldering though like maker kits, simple breadboard kits or maybe an older electronic component.

Since you admit to no prior experience, I fear that you might end up destroying the iPhone quickly. I would be more inclined to use it as a UI to an Arduino (or similar) embedded system.

Solder a FPGA to it's NAND pins and create virtual NAND flash device with humongous capacity. Next jailbreak and install tons of apps, specifically those that are at risk of being removed from the store. Never update, always be one version behind. Next: Advertise that you have access to all of that. People will be happy.

Sell it on Craigslist and buy a raspberry pi?