Importing into Lightroom you can choose to leave a file in its original location. Lightroom will create a thumbnail in the program. However all edits that you make to any picture are just a list of instructions. The original file is not adjusted. When you want to access a file with those edits--for printing/use outside of Lightroom, you 'export' the file which creates a copy to a new folder as assigned.
Not every time you open the program, but every few times, Lightroom will prompt you to test the integrity of Lightroom's library structure. So, for instance, if you use an external drive for some of the library, you would not want to test the integrity without that drive plugged into your computer or it will mark these files as missing. In the photo below you can see my Lightroom library. As you can see here, Lightroom is trying to access my iPhoto Library. It is not able to do so now that Photo has restricted the library, so it shows a "?" to alert to missing files. You can also see here my external drive, shaded to denote it is not currently available.

Generally speaking, my Canon 70D photos are never imported into iPhoto, but kept on my external drive. However, my older library resides in iPhoto/Photo and by default I have imported iPhone pictures into iPhoto/Photo then 'updated' the Lightroom library, which searches for all new additions and adds a link to them here as described above.
thanks