Step 1. If you installed PySide6

   a. In a venv then go to your venv's folder.
   b. Globally then go to your python installation folder.

Step 2. Go to Lib then site-packages then PySide6.

Step 3. Copy uic.exe ( or uic if the file extension are hidden) create a folder called bin and paste what you’ve copied inside it.

To compile ui files from:

QtDesigner: From the top menu select Form -> View Python Code… then click on the save icon (floppy disk) from the newly opened window.

Command Prompt: pyside6-uic.exe mainwindow.ui > ui_mainwindow.py

PowerShell: pyside6-uic.exe mainwindow.ui -o ui_mainwindow.py

If you using virtual environment, when you install pyside6 with pip in the virtual environment there is a folder named Scripts there is the pyside6-uic.exe tool.

if you have install pyside6 globally in your system and you use visual studio code you can use the extension PySide2-vsc then when you installed you can go to preferences > settings and search the PySide2-vsc extension settings then look for the “Command to compile a .ui file into python”. Then you can use that feature with right-click on .ui files.

The pyside6-uic tool is supposed to be installed automatically when installing the Python package.

Check if uic is in PATH

When using loadUiType, the Qt documentation (here) states that :

The internal process relies on uic being in the PATH. The pyside6-uic
wrapper uses a shipped uic that is located in the
site-packages/PySide6/uic, so PATH needs to be updated to use that if
there is no uic in the system.

But even then, I got the following error :

Cannot run 'pyside6-uic':  "execvp: No such file or directory"  -  
Exit status QProcess::NormalExit  ( 255 )
Check if 'pyside6-uic' is in PATH 

For me, pyside6-uic was not located in site-packages/PySide6/uic. When reinstalling the module with pip, I noticed this message :

  WARNING: The scripts pyside6-assistant, pyside6-designer, pyside6-genpyi, 
pyside6-linguist, pyside6-lrelease, pyside6-lupdate, pyside6-rcc and pyside6-uic are 
installed in '/Users/<user>/Library/Python/3.8/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, 
use --no-warn-script-location.

So make sure to add the right directory to your $PATH variable.
Once it’s done, you will be able to use the pyside6-uic command to generate a Python class from a UI file :

pyside6-uic mainwindow.ui > ui_mainwindow.py

Loading a .ui from code

You can also load a .ui file from your code using either:

1. loadUiType (doc page) :

This function generates and loads a .ui file at runtime, and it
returns a tuple containing the reference to the Python class, and the
base class.

2. or QUiLoader (doc page):

enables standalone applications to dynamically create user interfaces
at run-time using the information stored in UI files or specified in
plugin paths

from PySide6.QtUiTools import QUiLoader

ui_file = QFile("mainwindow.ui")
ui_file.open(QFile.ReadOnly)
loader = QUiLoader()
window = loader.load(ui_file)
window.show()

Generally speaking, use absolute paths to access your UI files. Relative paths are susceptible to errors.