> Is there really that much high-signal data that is not already being used?
Yes.
> Is their advantage in finding signals in data that other people overlook, or finding new data sources?
Yes.
I won't go into any particular detail, but there is a lot of signal in the market for those who are imaginative. The obvious and low hanging fruit is long gone, but there are still many places that offer an edge.
I wish there was some sort of full intro tutorial on finding strategies; ie: an example of a former signal (now traded away), the thought process, the data sourcing, statistical analysis, trading/signal strategy, etc..
The thing is that no one is really motivated to make a complete tutorial on finding strategies because it's economically irrational. You're either giving away specific sources of alpha or you're empowering potential competitors. This is why it's virtually guaranteed that anyone selling courses that teach trading or related skills is a fraud - they have essentially no incentive to just ramp up their own trading capital instead.
The industry is also extremely secretive (necessarily so). You'll hardly ever find a good treatise on finding novel signals, but there are tutorials on algo trading in general with examples of production strategies that used to work which have been, as you say, traded away. For that purpose I'd recommend you start here: http://www.decal.org/file/2945
It's public (technically it has to be in order to be strictly legal for use). But for the most part it is unintuitive, unclean (needs to be heavily normalized) and not easily accessible. There are a variety of vendors that source it, clean it and analyze it to make it salable to firms. Quantitative firms also have teams devoted to doing all of that internally.
Yes.
> Is their advantage in finding signals in data that other people overlook, or finding new data sources?
Yes.
I won't go into any particular detail, but there is a lot of signal in the market for those who are imaginative. The obvious and low hanging fruit is long gone, but there are still many places that offer an edge.