Maybe (integers only) something like the following......

Create a Text Property "input" to hold the current String of digits.

Create a Numeric Property "result" to hold the ongoing result of operations.

Create a section; add 10 digit buttons on it (0-9), 4 operations button (+,-,/,*) and 1 answer button (=).

Add in the 'Text Property' input - to display the current String of digits - make this readonly.

Configure the buttons to add their respective digit string to the 'input' property string.

(That is: an "Action Set" - of type 'click' - which should concatenate the relevant digit character to the existing 'input' property)

Build 4 Activities , add, subtract, multiply, divide - with two parameters 'input' and 'result'. (Not strictly necessary - you could operate *directly* on the 'input' and 'result' properties if you prefer)

Since each of these share some functionality - you might want to create a 5th helper Activity to check and convert the String to a integer.

As I type this - I realise you might need a third 'state' property to keep track of how to shift the data around and perform the relevant activity).

Note: this is just a suggestion - you might not even want to use a Text String here - you could deal with everything as an integer (but this will also have some implementation details to overcome).

For simplicity (in building, not using) your calculator - perhaps start with a 'Stack-Based' calculator (i.e implements Reverse Polish Notation)