Sadly, the functionality doesn't exist in macOS natively. There were some hacks to achieve it via SIMBL and a plugin called 'afloat' but that code hasn't been updated in about 6 years. There may be another newer App that can accomplish the goal. On newer macOS things are locked down rather tight and using such a tool would be kludgy at best and unreliable. That being said, various Apps can stay on top if they are hardcoded to enable the capability. But there is no system wide control over all windows to selectively force any window to always be on top.
Since you mentioned Terminal and since you are coming from Linux you should probably install iTerm2 as a replacement for the built-in Terminal app. It has many more features and capabilities.
One such feature with iTerm2 is the ability to configure a Hotkey Window setting on various iTerm2 profiles. iTerm2 -> Preferences -> Profile -> Keys tab -> Configure Hotkey Window button. Then set a hotkey to launch your Profile and make sure 'Pin hotkey window (stays open on loss of keyboard focus) is checked as well as 'Floating window'.
When you hit the hotkey it will launch iTerm2 and it will stay on top.
As to other Apps, such as Calculator. Did you know that Spotlight can do calculations? Try it out. Nifty feature. You can also put the Calculator on the widget sidebar. You could built an Automator script and hotkey to launch the Calculator.
It seems the paid PCalc app has a float on top option among many other features. Some other calculators are available on the App Store with similar capabilities. So it really depends on the App and if the developers coded the option.
