Yes, you can add the python binaries to PATH manually, but there may be a simpler option. In the first screen of the installer, there is a box that says "Add Python 3.9 to PATH":

For some reason, this box is not checked by default. So if you want pip and other binaries to be added to path during installation, you can just check this box.

So if you want, you can uninstall python (again) and reinstall it, this time making sure that box is check. However, if you want to just add the binaries to PATH manually, this is how you could go about doing that.

First, you need to get the path to the folder containing your binaries. To do that, navigate to your python installation, find the folder with the pip binaries, select it, and click "Copy path"

Now the path to that folder is copied to your clipboard.

Next you need to add that path to your PATH environment variable. To do that, follow these steps:

1. Go to start menu and type "path". Select the option that says "Edit system environment variables".

2. Click "Environment Variables..."

3. Scroll down and find "Path". Select it and click "Edit..."

4. Click "New"

5. Press Ctrl+v to paste the path you copied earlier. Hit enter.
6. Press Ok on each of the three open dialogues.

The pip binaries should now be in your path! Note that this procedure will work for adding any folder to PATH.