PCalc is a legendary application, started as a kind of "Hello World" non-trivial app on the Classic MacOS platform. Since those early days, there has been a PCalc release on every Apple device, using the native SDK.
Meanwhile, it has also gained some features. There are a few configurations to choose from, simple or scientific or RPN mode. You can edit the buttons.
It's a bit quirky, with a special mode for unit conversions, and a "tape" that shows the history of all the calculations, like a paper tape desktop calculator.
As a thing with buttons that helps you bang out a quick calculation, it doesn't suck.