PyQt5 UI is inconsistent across different platforms

Question

I'm trying to make a desktop application using pyqt5 because I need it to be cross platform but the UI is very inconsistent from a platform to another (the position of the widgets on the screen is different and sometimes they're on top of other widgets).

I'm new to pyqt5 and python in general so can you tell me why this is happening and how to fix it if possible?

PS: i'm using Qt designer to generate the UI and you can find all the code here

here are some examples:

on windows 10

on linux mint (ignore the missing images)