Typically an application is it's own process, though there are exceptions where apps can spin off their own processes (services). Broadcasts are the widely known form of IPC in Android. There's also traditional bs like shared memory, etc., as well as obscure low level stuff like implementing your own AIDL interface. But the ultimate trick is knowing that all the android specific stuff is essentially implementations on top of Binder, which does IPC through a kernel level thing that I don't understand.