Moving files from Macbook Pro to iPhone

Windows user needs help with file manipulation in the Apple environment.

I need to transfer files, while off-line, from my Macbook to my Iphone 7.

My source and target folders on each device is a sub-folder of the

program Caltopo.

Here is what I tried:

With the phone plugged in to the Mac I selected it from the sidebar and selected Files.

My program of interest, Caltopo, showed in the list of programs that support file sharing. 

I clicked the arrowhead to the left of Caltopo and opened up a list of files. In that list was a sub-folder called Tiles. This is my target folder both on the Mac and on the phone.

I opened a second Finder window and navigated the Mac’s folder system to Caltop's data folder and opened the sub-folder named Tiles.

I selected the files in the Mac’s Tiles folder and tried to drag them onto the iPhone’s Tiles folder. This is where I got stuck. I was unable to open the Tiles sub-folder and drop files onto it. I was only able to drop them onto the parent folder (named Caltopo).

Can anyone tell me how to get the files into the correct sub-folder?

I'm guessing that within the Apple environment you can’t manipulate and move files around as

easily as with Windows Explorer. Ie. You have to work via a restrictive interface. Or, more likely perhaps, I'm missing something.

Thanks!