This app was mentioned in 4 comments, with an average of 1.75 upvotes
Full disclosure, I'm an Android person and my goto was BostonBusMap before I moved out of state. Simple enough interface with several options for how you want your bus/train info displayed.
For the heck of it, I did a quick Googly, and found this +year old article for an app that is available for both iOS and Android.
FWIW, it's all the same API data, so none of them should give you wrong information. It really is just about finding your favorite interface and going with that. Try one and if it sucks, try another.
As an aside, feel lucky you have this freedom of choice because where I live now there is no subway and the buses don't have GPS transmitters on them. I'm well reacquainted with bus tables and I wait at stops with no real idea if it's gonna be 5 or 55 minutes til I'm moving. It's so nice to be back in 2007.
I used BostonBusMap in the past. Still have it, but since WFH I haven't needed it. Showed estimated bus times at stops or the actual bus locations. Never used with T or commuter rail so I can't comment on that aspect. Still appears to be getting periodic updates.
Gogole Maps for route planning, and Boston BusMap for bus times, they use the MBTA feed as well.
https://play.google.com/store/apps/details?id=boston.Bus.Map&hl=en