Source2mod is a tool that enables mods to run the old fashioned way without being published to the workshop. Instead of being loaded as an addon to Half-Life: Alyx, mods can be run with source2mod.exe using the -game parameter. Mods can be stored both inside the Half-Life: Alyx directory, and inside the source2mods folder in steamapps.
Download the latest version of source2mod here. Extract the contents of the compacted file to:
Mods can be run either by just executing source2mod.exe and manually selecting the mod, or by executing source2mod.exe with the -game parameter. For example:
source2mod.exe -game mysource2mod
Now you are able to play Half-Life Alyx standalone mods by running source2mod.exe