Android-using T riders can now get maps of the insides of some subway stops

I don't have a diagram, but it's pretty straightforward (you might want to look at a street map): the orange line was there first, and runs underneath Washington St. Due to the narrowness of the street, however, there wasn't enough room for the platforms to be directly opposite each other (as is the case for, e.g. Central Sq.). So the southbound tracks are south of Winter St., running along Washington St., and the northbound tracks are north of Summer St., also running along Washington St. A little further north at State St., the width was so limited that the tracks run one above the other.

A little later, they installed the red line underneath. The red line runs along Winter and Summer Sts., with all of the platforms under Summer St. Most of the interesting passages connecting the platforms or leading outside also parallel the red line tunnel. Above the tunnel are the Winter St. concourse connecting to Park St. and the Summer St. concourse (the long corridor reaching to Arch St., with the old connections to Filene's, etc. my understanding is that this passage originally ran all the way to South Stn. and was meant to carry trolleys between there and Park St., but it was never used for that. Probably for the best, as it would've had a four-way intersection with the orange line. In any case, the Dewey Sq. underpass from the original Artery cut through the passage. I don't know whether the Dig uses the same bit of tunnel, or if it goes under the Red line, not that it would matter much.

So basically, it's a cross shape, mostly under Washington St. and Summer St. with a few small passages branching off here and there.