On a standard, consumer Meta Quest headset, apps which have not been installed via the official Quest Store are located in subfolders in your App Library, notably "Unknown sources", "Company managed" or "For work"
As of mid-2023 Quest for Business is in beta testing; this removes the need for sideloading, or using a third party MDM tool
How to find the "For work" folder
How to find the "Company managed" folder
How to find the "Unknown sources" folder