For desktop, I use Visual Studio Code with the Python extension, which is fast, light, and easy to use. When I need it, PyCharm comes in handy with easy virtual environment setup and lots of easy to use configuration options. There are many more options, but VS Code and PyCharm are the most widely used ones, as far as I know.
PyDroid3 for Android has a really nice interface with lots of features, even without the paid liscense. Another option is QPython with the Python 3 plugin, which is slightly more complicated to install, but is completely free. (there is a QPython3 app which is dedicated to Python 3, but it seems as if the developer has stopped supporting it, or at least favors QPython with the plugin)
On iOS... sorry, but you're out of luck for offline apps.