To export a list of all symbols referenced in a map, you can use [Symbols] - [Rename & Reorder] - [Create symdef name file…]

Yes. Remy does that in his video Visiting Mars (go to the 1:15:23 mark):

@Rob_Wordsmith actually, Color Key is very simple: When the color matches the one specified, that area is "cut away".

And that's pretty much how it works. The rest is just figuring out how it interacts with other effects (ie: stuff like "Glow" will be calculated differently, since these "cut outs" introduce new edges).

You should also see this video where Remy goes through the process of changing a map style:

https://youtu.be/uqMbXZJbscA

That one in particular, is just the way Ross@2mtt created those assets. But CC3+ does offer filters, like transparency and blending modes, to achieve different effects.

I'm sure Remy did a video where he drew a cave with a pool and stuff underwater, but I can't find the video.

Yes, CC3+ has a huge potential because the tools are very flexible and powerful.

I'm... not sure... remember, I just started with CC3+ a couple of months ago. ;)

Actually, I'm not sure I'd choose to use gravel for a river bank. For example, I just added a "duff" texture and enabled blur for both the shoreline and the water to the quick test from above:

It might.

And if not, I'd still find a way to use it somewhere else (even if not for a river's bank).