Where do my iphone synced photos reside on my computer?

If they are on your computer, and they should be, they will be in the folder that is shown in this section of the LR preferences and also shown in the Folders section of the Library module left hand side panel.

In Classic, you can also right-click on any of the images (e.g. in the collection you mentioned) and select "Go to Folder in Library" and the source and LR will switch from showing the collection images to showing the folder images where the clicked on image lives.

Thanks!
2 follow-up questions:

1) So if I just used these auto-imported files, I would be OK resolution-wise? (There's no need, if I have this set-up, to import my iphone files a different way?)

2) I'm also wondering, what would happen if I wanted to move them to a hard drive? How do I do that without ruining something or breaking a connection/ is there something important I need to consider?

Yes, they'd be full resolution, and you can just drag them to a normal folder in the Folders panel without breaking anything.