Check out TripMode -- it's exactly for this problem. It lets you toggle which apps are allowed to use the network, plus it remembers which networks you wanted it to be enabled on. Super useful.
The only app I've used for Mac that "sort-of" does what your looking for is TripMode. However, I think the main purpose of TripMode is to track and limit data usage while tethering. There is nothing preventing you from keeping it turned on all the time merely for data usage tracking (after allowing all apps access to the internet while keeping TripMode on).
After a quick google search, PeakHour looks interesting and may do what you are interested in.
I'm not sure about an iPhone app that would help you. With data usage over WiFi - I know you can track cell network data from the settings app.
I use trip mode to manage cellular data usage on my MacBook Air. Handy when your laptop is tethered to a hotspot with a limited data cap, because macOS sometimes does a lot of stuff in the background, like sync photos and download software updates.
To encrypt your files use FileVault. And to make it a bit saver to use public hotspots I use Little Snitch, but its really advanced a simple and cheaper version will be Tripmode.
Hope it helps,
Could you use the tethering option on your iphone to leverage it's internet connection and just download straight to your PC? If so, I recommend the TripMode app to limit all other bandwith on your PC. I use it on mine when traveling and on spotty/slow connections: https://www.tripmode.ch/
On macOS I use TripMode to block apps while I am on a limited connection. Works great. Has a feature where you can set profiles for different networks, so for example no blocked apps while on your home wifi but blocks whichever apps while on a hotspot. $9 USD.
You can use any firewall to do this, though. Other options include Little Snitch (expensive but very capable, and wayyy overkill for just managing bandwidth) and Lulu (open source firewall similar to Little Snitch).
A simple little app to control which apps can or cannot have internets access based on individual wifi/hotspots/AP's.
I installed it a few weeks ago to prevent Backblaze backing up when I tether to my iPhone and it's easily the best $10 I've spent.
Check out a program called trip mode , I've used it on my mac and it sounds like exactly what you're looking for.
Allows you to select exactly what apps you want to use data and what apps you don't want, switches on as soon as you connect to the hotspot and switches off when you reconnect to another wifi network.
It costs like £10 or less but defiantly better than a big hotspot bill!
I use TripMode. Costs $7.99 though. https://www.tripmode.ch
You can see how much data you've used during a session (since TripMode was turned on), data from today, and data from the entire month. You can also limit which apps have internet access and see how much each app has used up so far (real-time updating).
I found the App I was looking for: TripMode.
An article about it: http://www.macworld.com/article/2923892/tripmode-review-tethered-mac-your-wallet-will-love-this-data-monitoring-app.html
Thanks, I wanted to update this post in case anyone else was searching for this.