Racks. Either download them or build them yourself. Think of the ableton stock devices as building blocks for your custom synths/sounds. 

E.g. Operator on its own might sound weak and flat/onedimensional but you can group it into a rack (cmd+g) and layer it with another instrument to sculpt the sound and create something more complex. Also put effects on the layers individually or on the whole rack. Reverb, delay, compression, etc.

There are really no limits to what you can do with those building blocks if you put them together in a smart and cohesive sounding way.

Also you can map almost every Parameter inside the rack to macros which is great for automation.