I don't know of any great resources, but from my personal experience and understanding of the landscape is that Fusion360 is the most common real CAD program used by hobbyists, and small shops (especially those that are basically "hobbyist bought a bigger mill and moved into a larger garage"). Some of the bigger places will use SolidWorks (which is what 95% of my CAD experience is in), but the price there is way out of the range of someone just trying to learn.
In general, the aspect you're asking about is called an "assembly" - where you can bring in multiple parts that you have designed (and even design new parts "in place") to see how they go together and, to a minor extend, interact. I say "minor extent" there because most assembly systems aren't running full physics simulations or collision detection, at least most of the time - SolidWorks will happily let you design and assemble a model that is physically impossible to put together, while letting you rotate bodies through each other.
So yeah, I'd recommend starting with Fusion360. There are plenty of resources out there for learning it, but I do know that Grimsmo Knives and NYC CNC have videos showing how they specifically use it.
Here's what seems like a very in-depth video from Grimsmo on 5-axis machining (so definitely not applicable to just starting to learn, but this is the first I found - I know plenty of their other videos have details about fixturing and setup): https://www.youtube.com/watch?v=XqhctiVZtRU
NYC CNC has an entire playlist called "Fusion 360 for Beginners": https://www.youtube.com/nyccnc/playlists ; I haven't watched any of that, but I've watched plenty of their videos and enjoyed them, so think that playlist should be at least a bit helpful.
There's plenty more detail here, but I don't have the time at the moment to dig deeper - if you have any questions, feel free to leave them here and I'll see what I can dig up.