Well, yes, ask LIGO, but it's not that hard, and people in PSE can give you some answers.
But, no need to ask, they've published some and there are other papers and more detailed summaries that you can google. For LIGO you can start with https://www.ligo.caltech.edu/page/gw-sources. Or google gravitational wave sources
Yes, of course gravitational waves superimpose, but weak waves superimposed on stronger waves can be unnoticeable. Like electromagnetic waves, in the linear approximation the propagate and decay with distance as 1/r far enough away. The strongest gravitational waves are emitted by very dense and very small object, such as black holes and neutron stars, when they are strongly perturbed such as in collision or merger. There's other strong sources likely from the early days of the universe, before recombination, that we would love to build big enough detectors to see their likely longer wavelengths.
We don't see the sum of all that because we don't have enough sensitivity, and because their frequencies can be different (as with electromagnetic waves they superimpose at the same frequency, the more nonlinear effects are much weaker).
The nonlinear regime for gravitational waves is where things get much more complicated, because gravitation, in General Relativity, essentially interacts with itself -- gravitational effects interfere with each other if strong enough.
Another factor that is interesting for gravitational waves is that they can not be produced by monopoles (single masses), or even dipole distributions, but can by quadrupoles and n-poles, for higher $2^n$. Most (or all) we've detected so far has been from the mergers happening not exactly radially so they have a quadrupole moment (think of a barbell rotating in any axis other than its axis of symmetry). If we get more sensitive detectors we'll see more and effects from other gravitational distortions.
The astrophysical and physics community is looking to build more sensitive and bigger detectors of gravitational waves. They are looking at those as basically being gravitational observatories, a whole new regime to explore the universe. One paper on that is for instance at https://arxiv.org/pdf/1209.0667.pdf. I'm sure there are more recent papers that detail what specifically could be seen for each size of interferometer.