Install Python on linux is very straightforward as package for PyQt5 are available in the repositories of most distributions. In raspbian you can install either from the command line or via “add/remove software”. the packages you are looking for are named python3-pyqt5 depending on which version you are installing for.
you can also install these from the command line as follows:
sudo apt-get install python3-pyqt5
and here when you install from “add/remove Software”
for creating apps using programmatical python code, that is fine for simple applications, but as your application get larger or interface become more complicated, it can get a bit cumbersome to define all element programmatically. The good news is that Qt comes with a graphical editor – Qt Creator which contains a drag-and-drop UI editor.
you can install Qt-Creator using command line
sudo apt-get install qt5-default
sudo apt-get install qtcreator