It’s not an app, it’s a web page. BigTreesTech does however have some technology behind the curtain that presents a phone friendly interface when it detects that it’s connected to a phone (no user intervention required).
Just use phone browser apps like safari, firefox, chrome or whatever to go to http://www.bigtreestech.com
FYI, while the pictures have been updating I noticed that one of the cameras seems to have been damaged a bit by the heavy snow we had and is pointing at an odd angle. I’ll try to get that fixed soon but it does still provide several views of the drive way and empty lot next door.