Although always nice to learn by doing it yourself, but also it is great fun to build on and extend on what is already available. Checkout this module created by Intelizign: https://marketplace.mendix.com/link/component/118700/Intelizign/Calculator. It is built in 9.5, probably doing fine there, but in 9.11 it looks pretty different:
You can build a basic-level calculator app here on Mendix by input through the keyboard in a single input field: you'll require to parse it. Just input using the mouse: e.g. tap '3' (-> store as 'temp') | tap'+' (-> store this as 'operator' + store '3' as 'operand1') | click '1' (-> store as 'temp') | press '5' -> 'temp' now becomes 15 (10*1+5)) | click '=' (do the real calculation depends on what you kept already. Obviously, if you need to execute some different and more technical tasks, then the method will vary. Apart from that, from the above method, users can do it easily on the Mendix platform.