There are a few different ways administrators can accomplish this; the two most common are likely via GPO settings or through a layout modification XML file pushed through GPO.
The local group policy editor has a Start Menu and Taskbar section within User Configuration that has all kinds of options:
You can get really into specifics through the XML method, with a file that looks like this:
<?xml version="1.0" encoding="utf-8"?>
<LayoutModificationTemplate
xmlns="http://schemas.microsoft.com/Start/2014/LayoutModification"
xmlns:defaultlayout="http://schemas.microsoft.com/Start/2014/FullDefaultLayout"
xmlns:start="http://schemas.microsoft.com/Start/2014/StartLayout"
xmlns:taskbar="http://schemas.microsoft.com/Start/2014/TaskbarLayout"
Version="1">
<CustomTaskbarLayoutCollection>
<defaultlayout:TaskbarLayout>
<taskbar:TaskbarPinList>
<taskbar:UWA AppUserModelID="Microsoft.MicrosoftEdge_8wekyb3d8bbwe!MicrosoftEdge" />
<taskbar:DesktopApp DesktopApplicationLinkPath="%APPDATA%\Microsoft\Windows\Start Menu\Programs\System Tools\File Explorer.lnk" />
</taskbar:TaskbarPinList>
</defaultlayout:TaskbarLayout>
</CustomTaskbarLayoutCollection>
</LayoutModificationTemplate>
Where you can see there is a specific section for <taskbar:TaskbarPinList>
.