Although the appearance of the calculator has changed little from Lollipop, and some of the UI code is indeed the same, the rest of the code has changed substantially. Unsurprisingly, Calculator.java implements the main UI. The other major parts of the implementation are:
CR.java in external/crcalc provides the underlying demand-driven ("constructive real") arithmetic implementation. Numbers are represented primarily as objects with a method that can compute arbitrarily precise approximations. The actual arithmetic performed by these methods is based on Java's java.util.BigInteger arithmetic, with appropriate implicit scaling.
BoundedRational.java is a rational arithmetic package that is used to provide finite exact answers in "easy" cases. It is used primarily to determine when an approximation provided by CR.java is actually exact. This is used in turn both to limit the length of displayed results and scrolling, as well as to identify errors such as division by zero, that would otherwise result in timeouts during computations. It is in some sense not needed to produce correct results, but it significantly improves the usability of the calculator. It is also used for the "display as fraction" option in the overflow menu.
CalculatorExpr.java implements calculator arithmetic expressions. It supports editing, saving, restoring, and evaluation of expressions. Evaluation produces a constructive real (CR) and possibly a BoundedRational result. Unlike the "arity" library used in earlier versions, the underlying expression is represented as a sequence of "tokens", many of which are represented by Button ids, not as a character string.
Evaluator.java implements much of the actual calculator logic, particularly background expression evaluation. Expression evaluation here includes both using CalculatorExpr.java to evaluate the expression, and then invoking the resulting CR value to actually produce finite approximations and convert them to decimal. Two types of expression evaluation are supported: (1) Initial evaluation of the expression and producing an initial decimal approximation, and (2) reevaluation to higher precision. (1) is invoked directly from the Calculator UI, while (2) is invoked from the calculator display, commonly in response to scrolling. When the display requests a result, a "result" is immediately returned, though it may contains blank placeholders. The display is then notified when the real result becomes available.
CalculatorText.java is the TextView subclass used to display the formula.
CalculatorResult.java is the TextView subclass used to display the result. It handles result formatting, scrolling, etc. After the user hits "=", the CalculatorResult widget moves into the top position, replacing the formula display. Currently it remains in that position until the formula is again modified.