Half-Life: Alyx Workshop Tools/Porting Source 1 Content to Source 2
Source 1 models, materials, maps, and all of it's content is completely different to how Source 2 handles the same files. These files are now unsupported, but with some built-in tools and some know-how, you can learn to import Source content to Source 2.
Some Context and Preamble
Source 2 uses new file formats for it's content. Maps are now .VMAP instead of .VMF, materials are now .VMAT instead of .VMT, models are now .VMDL instead of .MDL. See the pattern yet? Almost every file used in Source 1 is now some form of .v___. Some of them haven't changed much, like .VCFG files are just old-school .CFG files from Source 1 renamed. But what is unique about these files is that for models, maps, materials, sounds, etc. they all use a compiled version of said file.
Take for instance the .vmat file. If you open a .vmat file inside of your 'Half-Life Alyx/content' directory, it will be in plain-text and can be edited. However, find that same material in 'Half-Life Alyx/game' and it will become a .VMAT_C file. Think of .VMAT as just a plain-text version of .VMAT_C, which is the real material file loaded by the engine. So, if you're making new content or have the sources of the Source 1 content, you should probably just convert your content using the regular paths, as it's much easier to retain your ideal look with the new GUI tools Source 2 offers.
Guides For Converting Content
Valve has an internal tool to help facilitate Source 1 content to Source 2, but we unfortunately are not given access to this tool. However, the community has come up with some clever workarounds using the conversion processes available in the engine and tools itself.
If you'd like to port over your own content to Source 2, modeling and material creation guides are here: