The latest info about recommended Unity versions is available in the Snapdragon Spaces documentation at https://docs.spaces.qualcomm.com/unity/setup/SetupGuideUnity.html#prerequisites.
Snapdragon Spaces requires Unity 2021 or later versions. The recommended version is Unity 2021.3.16 which we also use internally. This means it is tested most frequently and hence most stable. You can find it in the Unity Download Archive (unity.com). While using Unity 2021.3.16 is recommended, you should be able to use other 2021.3+ versions as well.
Previously we recommended all versions 2020+ which are not fully supported anymore. When opening the QCHT (Qualcomm Hand Tracking) samples with Unity 2020, the console is going to show the following error: "The type or namespace name 'IPoinerMoveHandler' could not be found (are you missing a using directive or an assembly reference?)"
This is caused by a dependency issue between Unity, the XR Interaction Toolkit (XRIT), and QCHT.