Source 2 Filmmaker Theory
Transitioning from Source 1 to Source 2, especially when used to the Filmmaker, can be a bit of a difficult pill to swallow. This guide is an messy attempt to make that process a little bit easier to stomach. There are some things that are just different about Source 2, but can prove rewarding if you utilize them the way they were intended to, rather than using an old workflow and adapting it to a new tool!
Part 0: Lighting and Workshop Workarounds
Getting started with Source 2, you will have to perform some workarounds to do the things you're used to, like manipulating lights or getting content and assets from the Half-Life: Alyx Workshop.
Part 1: Source 1 vs. Source 2 - A Briefing
Source 2 uses a new content pipeline, which is built off of Valve's content system they've been using internally since Source 1. All of the raw files that make up the game's content, such as .fbx/.dmx files, .tga/.png/.psd, and other sources are stored inside of the game's "content" directory, found in the Half-Life: Alyx directory next to "game".
"game" is a more familiar file structure, this is where "compiled" content exists, so it is where you will find Valve's new file formats like .vmdl_c, .vmat_c, .vtex_c, and .vpk, which can either store map data, if a .vpk is inside "hlvr/maps", or game content if it's inside a pakXX.vpk file.
So there's a lot of confusing information there, but essentially, when you're making content like models and materials, they exist in "content" and if you're just using them in SFM or Hammer, that content exists in "game".
Valve uses .vmap, .vmdl, .vmat, and more files of that nature inside "content" to store the text data used to compile into the game. These usually have analogs to old file formats from Source 1, such as .vmdl being analogous to the .qc files, and .vmdl_c being analogous to .mdl.
Part 2: Make Your Own Maps!
Coming from Source 1, you may be expecting to go into a retail or workshop map and just start building a scene around that. For most cases, this is good, however it does limit you when you want to utilize some of the advanced graphical techniques available in Source 2. The biggest one most people will be familiar with is volumetric lighting, which used to be available on a per-light basis, but is now defined by the map you are making videos in.
The other big feature is baked lighting. Where some modern engines are concerning themselves with realtime global illumination and raytracing, Valve considers baked lighting to be the best for their VR, performance-driven platform used in Half-Life: Alyx. So Half-Life: Alyx uses a new, still work-in-progress system called VRAD3, which is built off of Valve's old VRAD and VRAD2 baking processes. It's slow, but creates beautiful maps and is why Alyx has the upper hand in VR fidelity right now.
For this reason, it is recommended that users of the S2FM get acquainted with building maps on their own in Hammer. This can have a lot of benefits, most importantly the ability to import model files from other programs directly into the map as world geometry. No need to import every single model from another game, you can now just import world geometry directly into Hammer and modify it manually using Hammer's new mesh-based modeling tools! You could even import the whole map from some other game!
Also, Source 2's Hammer being a fully fledged modeling program has it's own extra bits of flair. You can model something entirely using Hammer's tools, then export that as a .vmdl file,
Other than that, if you're used to scene-building in Source 1 Filmmaker, you will find that Hammer is much more suitable for this task. And while documentation is still coming out for it, the community has put together a ton of resources to make maps look good.
Part 3: Port Content Yourself!
Source Filmmaker's ubiquity in the machinima world is due to the fact that Source 1 was an intensely modifiable engine with almost 15 years of documentation and community to go along with it. As Source 2 is still in it's early stages, I think you may find yourself a little bit concerned with the lack of content available in Source Filmmaker. However, the greatest improvement in Source 2 is not it's graphical fidelity, but rather it's brand new pipeline for custom content. Valve's new Material Editor, ModelDoc, and Hammer have a ton of potential to easily port content over from other games.
Source 2 also includes some pipelines for converting Source 1 content to Source 2.
So, learn the tools! Learn to port content yourself! Source 2 is a bit of a weird beast, but learning to utilize it's fullest potential can reward you with much better looking films and a stronger workflow. It's not just a plaything for machinima anymore!